update drop_range API to use RangeBounds

[zaidoon1]

to address #148 (comment)

[zaidoon1]

linter failure is related to:

error: the variable `count` is used as a loop counter
   --> src/tree/mod.rs:149:9
    |
149 |         for (key, value) in iter {
    |         ^^^^^^^^^^^^^^^^^^^^^^^^ help: consider using: `for (count, (key, value)) in iter.enumerate()`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#explicit_counter_loop

    

but I did't touch this code?

[marvin-j97]

linter failure is related to:

error: the variable `count` is used as a loop counter
   --> src/tree/mod.rs:149:9
    |
149 |         for (key, value) in iter {
    |         ^^^^^^^^^^^^^^^^^^^^^^^^ help: consider using: `for (count, (key, value)) in iter.enumerate()`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#explicit_counter_loop

    

but I did't touch this code?

I think the linter rules changed again - I updated the 3.0.0 branch to allow it.