made boost::unordered_[multi](map|set)::[const_][local_]iterator instantiable with incomplete value_type

[joaquintides]

Addresses #319.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.23%. Comparing base (4b76ac8) to head (eba3e85).
⚠️ Report is 1 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #321   +/-   ##
========================================
  Coverage    98.23%   98.23%           
========================================
  Files          161      161           
  Lines        22223    22239   +16     
========================================
+ Hits         21831    21847   +16     
  Misses         392      392           

Files with missing lines	Coverage Δ
include/boost/unordered/detail/fca.hpp	99.35% <100.00%> (ø)
include/boost/unordered/detail/implementation.hpp	99.23% <ø> (ø)
test/unordered/incomplete_test.cpp	73.33% <100.00%> (+14.71%)	⬆️

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