Permit building coder maps ahead-of-time of TypeMap init

Building coder maps can be quite expensive, building them ahead-of-time
allows them to be reused for multiple type map creations, e.g. if
creating both BasicTypeMapForQueries and BasicTypeMapForResults the cost
need only be paid once.

Example usage:

    conn = PG.connect(…)
    coder_maps = BasicTypeRegistry.build_coder_maps(conn)
    tmq = BasicTypeMapForQueries.new(conn, coder_maps: coder_maps)
    tmr = BasicTypeMapForResults.new(conn, coder_maps: coder_maps)

Testing methodology:

- 189k rows returned from pg_type query in `build_coder_maps`.
- Cached and injected results from query to remove query cost and
  jitter from baseline.
- Timing calling `PG::BasicTypeMapForQueries.new` only, figures are
  average of 32 iterations.
- “Cached” calls `build_coder_maps` ahead-of-time, outside the timing
  loop, and passes-in.

          runtime
Baseline:  374±16 ms
Cached  :   <1± 0 ms (-99%)

This confirms that the majority of the cost in creating a new type map
is, when ignoring SQL query and results materialization, enclosed within
`build_coder_maps`. As such, for *n* type maps, this reduces the
(sometimes non-small) time complexity effectively from O(n) → O(1).

lib/pg/basic_type_mapping.rb

@@ -87,8 +87,6 @@ def coder_by_oid(oid)
 		end
 	end
 
-	private
-
 	def build_coder_maps(connection)
 		result = connection.exec(<<-SQL).to_a
 			SELECT t.oid, t.typname, t.typelem, t.typdelim, ti.proname AS typinput
@@ -107,6 +105,7 @@ def build_coder_maps(connection)
 			h
 		end
 	end
+	module_function :build_coder_maps
 
 	ValidFormats = { 0 => true, 1 => true }
 	ValidDirections = { :encoder => true, :decoder => true }
@@ -122,8 +121,6 @@ def check_format_and_direction(format, direction)
 	# objects as values.
 	CODERS_BY_NAME = []
 
-	public
-
 	# Register an encoder or decoder instance for casting a PostgreSQL type.
 	#
 	# Coder#name must correspond to the +typname+ column in the +pg_type+ table.
@@ -290,8 +287,8 @@ def typecast_result_value(result, _tuple, field)
 		end
 	end
 
-	def initialize(connection)
-		@coder_maps = build_coder_maps(connection)
+	def initialize(connection, coder_maps: nil)
+		@coder_maps = coder_maps || build_coder_maps(connection)
 
 		# Populate TypeMapByOid hash with decoders
 		@coder_maps.flat_map{|f| f[:decoder].coders }.each do |coder|
@@ -382,8 +379,8 @@ class BinaryData < String
 
 	include PG::BasicTypeRegistry
 
-	def initialize(connection)
-		@coder_maps = build_coder_maps(connection)
+	def initialize(connection, coder_maps: nil)
+		@coder_maps = coder_maps || build_coder_maps(connection)
 		@array_encoders_by_klass = array_encoders_by_klass
 		@encode_array_as = :array
 		init_encoders

spec/pg/basic_type_mapping_spec.rb

@@ -635,4 +635,12 @@ def to_s
 			end
 		end
 	end
+
+	describe PG::BasicTypeRegistry do
+		it "can build coder maps" do
+			expect do
+				described_class.build_coder_maps(@conn)
+			end.to_not raise_error
+		end
+	end
 end