erase(iterator) perf

[cmazakas]

Add a dedicated erase_node() function to the table class used internally by the implementation, creating optimizer-friendly code for cl.exe.

Partially addresses #137

[codecov]

Codecov Report

Merging #138 (a31e894) into develop (3abe5de) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop     #138      +/-   ##
===========================================
- Coverage    97.28%   97.28%   -0.01%     
===========================================
  Files           75       75              
  Lines         9943     9941       -2     
===========================================
- Hits          9673     9671       -2     
  Misses         270      270              

Impacted Files	Coverage Δ
include/boost/unordered/detail/implementation.hpp	98.57% <100.00%> (+0.01%)	⬆️
include/boost/unordered/unordered_map.hpp	99.84% <100.00%> (-0.01%)	⬇️
include/boost/unordered/unordered_set.hpp	98.90% <100.00%> (-0.01%)	⬇️

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