Support key function

[yurrriq]

I found myself needing a key function for bisect_right and noticed this library didn't support it so I had to write my own. I figured I'd see if you're interested.

I went ahead and added the key functionality to bisect_left, insort_left, and insort_right, too.

[codecov]

Codecov Report

Attention: Patch coverage is 64.28571% with 10 lines in your changes missing coverage. Please review.

Project coverage is 78.57%. Comparing base (0b48f2f) to head (72fe606).

Files with missing lines	Patch %	Lines
src/bisect.jl	60.00%	8 Missing ⚠️
src/insort.jl	75.00%	2 Missing ⚠️

❗ There is a different number of reports uploaded between BASE (0b48f2f) and HEAD (72fe606). Click for more details.

HEAD has 7 uploads less than BASE

Flag	BASE (0b48f2f)	HEAD (72fe606)
	16	9

Additional details and impacted files

@@             Coverage Diff             @@
##           master      #13       +/-   ##
===========================================
- Coverage   95.00%   78.57%   -16.43%     
===========================================
  Files           4        4               
  Lines          40       56       +16     
===========================================
+ Hits           38       44        +6     
- Misses          2       12       +10     

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[yurrriq]

If you're amenable to adding this functionality, please let me know and I'll happily add tests.

[singularitti]

Hi @yurrriq, I just saw you made this PR. Sorry for the late reply! I will have a look soon!