Add identity API

[arximboldi]

I decided not to change the API of update_if_exists to return a bool. This could however maybe help.

FYI @omer-s @harryhk

[codecov-commenter]

Codecov Report

Merging #238 (2c8ceb6) into master (ca78d68) will increase coverage by 0.01%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #238      +/-   ##
==========================================
+ Coverage   90.48%   90.50%   +0.01%     
==========================================
  Files         119      119              
  Lines       12092    12109      +17     
==========================================
+ Hits        10942    10959      +17     
  Misses       1150     1150              

Impacted Files	Coverage Δ
immer/array.hpp	100.00% <100.00%> (ø)
immer/flex_vector.hpp	100.00% <100.00%> (ø)
immer/map.hpp	99.01% <100.00%> (+<0.01%)	:arrow_up:
immer/set.hpp	100.00% <100.00%> (ø)
immer/vector.hpp	100.00% <100.00%> (ø)
test/map/generic.ipp	99.13% <100.00%> (+<0.01%)	:arrow_up:
test/set/generic.ipp	99.30% <100.00%> (+<0.01%)	:arrow_up:
test/vector/generic.ipp	99.67% <100.00%> (+<0.01%)	:arrow_up:

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