Emphasizing (non-)mutating behaviour by using bang-bang notation

[devmotion]

To emphasize that value! etc. are sometimes mutating, sometimes non-mutating, I suggest adopting the bang-bang notation (see https://github.com/JuliaFolds2/BangBang.jl) and renaming these functions to value!! etc.

I also added explicit warnings to each docstring.

Closes https://github.com/JuliaDiff/DiffResults.jl/issues/17. Closes https://github.com/JuliaDiff/DiffResults.jl/issues/4.

[codecov[bot]]

Codecov Report

:x: Patch coverage is 84.00000% with 4 lines in your changes missing coverage. Please review. :white_check_mark: Project coverage is 86.84%. Comparing base (0479a8a) to head (1a9c174).

Files with missing lines	Patch %	Lines
src/DiffResults.jl	84.00%	4 Missing :warning:

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #39   +/-   ##
=======================================
  Coverage   86.84%   86.84%           
=======================================
  Files           1        1           
  Lines          76       76           
=======================================
  Hits           66       66           
  Misses         10       10           

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