Support named fields for field indexing in messages

[PragmaTwice]

Finished #1.

[codecov-io]

Codecov Report

Merging #6 (7105a24) into master (7eaad36) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master        #6   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           19        19           
  Lines          463       477   +14     
=========================================
+ Hits           463       477   +14     

Impacted Files	Coverage Δ
include/protopuf/field.h	100.00% <ø> (ø)
test/coder.cpp	100.00% <ø> (ø)
include/protopuf/message.h	100.00% <100.00%> (ø)
test/message.cpp	100.00% <100.00%> (ø)

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