Add get_mut() to maps

[arthurprs]

With it the user can avoid the clunky pattern get() + clone + modify + insert(). Especially useful if you have early returns (eg. with ?) in between.

[codecov-io]

Codecov Report

Merging #52 (e8bed47) into master (60a2f9c) will decrease coverage by 0.40%. The diff coverage is 75.00%.

@@            Coverage Diff             @@
##           master      #52      +/-   ##
==========================================
- Coverage   93.24%   92.83%   -0.41%     
==========================================
  Files          11       11              
  Lines        1583     1620      +37     
==========================================
+ Hits         1476     1504      +28     
- Misses        107      116       +9     

Impacted Files	Coverage Δ
src/map/entry.rs	100.00% <ø> (ø)
src/map/red_black_tree_map/mod.rs	92.81% <55.55%> (-0.67%)	:arrow_down:
src/map/hash_trie_map/mod.rs	91.78% <78.57%> (-0.94%)	:arrow_down:
src/list/mod.rs	91.66% <100.00%> (+0.19%)	:arrow_up:

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[orium]

Merged. Thanks.

[arthurprs]

I was hoping you could make a new release? That will include the rand 0.8 update and the get_mut. Thank you :pray:

[orium]

Sure. Released v0.9.0.