Tuning code – Fixes #36

[NelsonVides]

This PR again tunes code here. Removing some bug verification not needed since OTP20, and also removing copying of atoms, makes the object code 10% smaller ( exml_nif.so goes from 1MB to 909KB), which adds to the fact that there's a bit less to compute, and improves throughput for the parser by 10%. Which btw, copying (attempting to copy!) atoms should have never been necessary, atoms are global and they don't depend whatsoever in the environment in which they're added, so the enif_make_copy/2call would do nothing more than checking the atom is already available in the global atom table, which is a waste of checks.

Also adding prettier docs to the hex package.

Fixes #36

[codecov-commenter]

Codecov Report

Merging #58 (0d9d339) into master (3a88d98) will decrease coverage by 0.26%.
The diff coverage is 95.23%.

@@            Coverage Diff             @@
##           master      #58      +/-   ##
==========================================
- Coverage   71.76%   71.49%   -0.27%     
==========================================
  Files           7        7              
  Lines        1119     1098      -21     
  Branches      160      158       -2     
==========================================
- Hits          803      785      -18     
+ Misses        316      313       -3     

Impacted Files	Coverage Δ
src/exml.erl	91.42% <ø> (ø)
src/exml_nif.erl	41.66% <ø> (ø)
c_src/exml.cpp	94.68% <95.23%> (+0.29%)	⬆️
c_src/rapidxml.hpp	60.54% <0.00%> (+0.12%)	⬆️

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[chrzaszcz]

Looks good!