Add basic OpenStruct benchmarks

All of the benchmarks except for 100x reads show a performance
improvement.  If you are reading the data repeatedly, it's still much
better to use Struct or Data.

```
$ benchmark-driver benchmarks/ostruct.yml
Warming up --------------------------------------
                  new     7.047k i/s -      7.711k times in 1.094212s (141.90μs/i)
      attr_write_read     4.904k i/s -      4.910k times in 1.001289s (203.93μs/i)
index_write_attr_read     6.346k i/s -      6.963k times in 1.097167s (157.57μs/i)
           null_reads    79.488k i/s -     85.250k times in 1.072495s (12.58μs/i)
            10x_reads     6.651k i/s -      7.282k times in 1.094836s (150.35μs/i)
           100x_reads     5.136k i/s -      5.150k times in 1.002780s (194.71μs/i)
Calculating -------------------------------------
                          v0.6.0       local
                  new     6.938k     33.067k i/s -     21.141k times in 3.047342s 0.639334s
      attr_write_read     4.886k     14.021k i/s -     14.711k times in 3.010686s 1.049232s
index_write_attr_read     6.270k     20.225k i/s -     19.039k times in 3.036489s 0.941373s
           null_reads    78.351k     77.187k i/s -    238.462k times in 3.043491s 3.089414s
            10x_reads     6.518k     13.306k i/s -     19.953k times in 3.061390s 1.499576s
           100x_reads     4.454k      2.078k i/s -     15.407k times in 3.459125s 7.414447s

Comparison:
                               new
                local:     33067.2 i/s
               v0.6.0:      6937.5 i/s - 4.77x  slower

                   attr_write_read
                local:     14020.7 i/s
               v0.6.0:      4886.3 i/s - 2.87x  slower

             index_write_attr_read
                local:     20224.7 i/s
               v0.6.0:      6270.1 i/s - 3.23x  slower

                        null_reads
               v0.6.0:     78351.5 i/s
                local:     77186.8 i/s - 1.02x  slower

                         10x_reads
                local:     13305.8 i/s
               v0.6.0:      6517.6 i/s - 2.04x  slower

                        100x_reads
               v0.6.0:      4454.0 i/s
                local:      2078.0 i/s - 2.14x  slower
```

benchmarks/ostruct.yml

@@ -0,0 +1,154 @@
+prelude: |
+  # frozen_string_literal: true
+
+  require "ostruct"
+  keys = (0..29).map { :"method_#{_1}" }
+  input = keys.to_h { [_1, _1] }
+
+benchmark:
+  new: |
+    OpenStruct.new(input)
+
+  attr_write_read: |
+    ostruct = OpenStruct.new
+    ostruct.method_0 = "foo"
+    ostruct.method_1 = "bar"
+    ostruct.method_2 = "baz"
+    ostruct.method_3 = "quux"
+    ostruct.method_4 = "foo"
+    ostruct.method_5 = "bar"
+    ostruct.method_6 = "baz"
+    ostruct.method_7 = "quux"
+    ostruct.method_8 = "quux"
+    ostruct.method_9 = "quux"
+    ostruct.method_10 = "foo"
+    ostruct.method_11 = "bar"
+    ostruct.method_12 = "baz"
+    ostruct.method_13 = "quux"
+    ostruct.method_14 = "foo"
+    ostruct.method_15 = "bar"
+    ostruct.method_16 = "baz"
+    ostruct.method_17 = "quux"
+    ostruct.method_18 = "quux"
+    ostruct.method_19 = "quux"
+    ostruct.method_20 = "foo"
+    ostruct.method_21 = "bar"
+    ostruct.method_22 = "baz"
+    ostruct.method_23 = "quux"
+    ostruct.method_24 = "foo"
+    ostruct.method_25 = "bar"
+    ostruct.method_26 = "baz"
+    ostruct.method_27 = "quux"
+    ostruct.method_28 = "quux"
+    ostruct.method_29 = "quux"
+    ostruct.method_0
+    ostruct.method_1
+    ostruct.method_2
+    ostruct.method_3
+    ostruct.method_4
+    ostruct.method_5
+    ostruct.method_6
+    ostruct.method_7
+    ostruct.method_8
+    ostruct.method_9
+    ostruct.method_10
+    ostruct.method_11
+    ostruct.method_12
+    ostruct.method_13
+    ostruct.method_14
+    ostruct.method_15
+    ostruct.method_16
+    ostruct.method_17
+    ostruct.method_18
+    ostruct.method_19
+    ostruct.method_20
+    ostruct.method_21
+    ostruct.method_22
+    ostruct.method_23
+    ostruct.method_24
+    ostruct.method_25
+    ostruct.method_26
+    ostruct.method_27
+    ostruct.method_28
+    ostruct.method_29
+
+  index_write_attr_read: |
+    ostruct = OpenStruct.new
+    keys.each_with_index do ostruct[_1] = _2 end
+    keys.each do ostruct.send(_1) end
+
+  index_write_attr_read: |
+    ostruct = OpenStruct.new
+    keys.each_with_index do ostruct[_1] = _2 end
+    keys.each do ostruct.send(_1) end
+
+  null_reads: |
+    ostruct = OpenStruct.new
+    ostruct.method_0
+    ostruct.method_1
+    ostruct.method_2
+    ostruct.method_3
+    ostruct.method_4
+    ostruct.method_5
+    ostruct.method_6
+    ostruct.method_7
+    ostruct.method_8
+    ostruct.method_9
+    ostruct.method_10
+    ostruct.method_11
+    ostruct.method_12
+    ostruct.method_13
+    ostruct.method_14
+    ostruct.method_15
+    ostruct.method_16
+    ostruct.method_17
+    ostruct.method_18
+    ostruct.method_19
+    ostruct.method_20
+    ostruct.method_21
+    ostruct.method_22
+    ostruct.method_23
+    ostruct.method_24
+    ostruct.method_25
+    ostruct.method_26
+    ostruct.method_27
+    ostruct.method_28
+    ostruct.method_29
+
+  10x_reads: |
+    ostruct = OpenStruct.new(input)
+    10.times do
+      ostruct.method_0
+      ostruct.method_1
+      ostruct.method_2
+      ostruct.method_3
+      ostruct.method_4
+      ostruct.method_5
+      ostruct.method_6
+      ostruct.method_7
+      ostruct.method_8
+      ostruct.method_9
+    end
+
+  100x_reads: |
+    ostruct = OpenStruct.new(input)
+    100.times do
+      ostruct.method_0
+      ostruct.method_1
+      ostruct.method_2
+      ostruct.method_3
+      ostruct.method_4
+      ostruct.method_5
+      ostruct.method_6
+      ostruct.method_7
+      ostruct.method_8
+      ostruct.method_9
+    end
+
+contexts:
+  - name: v0.6.0
+    gems:
+      ostruct: 0.6.0
+  - name: local
+    prelude: |
+      $LOAD_PATH.unshift "./lib"