Message fields can have hex field numbers

[CantelopePeel]

This change allows fields in messages to have hex field numbers. Previously a message of the following form would have raised an error:

message A {
    required uint32 b = 0x1234;
}

Added unit tests to test parsing of both decimal and hex field numbers.

Question for maintainer: Are there other places where I can propagate this in a followup?

[codecov[bot]]

Codecov Report

Merging #214 (42a8352) into master (3802b81) will increase coverage by 0.14%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #214      +/-   ##
==========================================
+ Coverage   88.86%   89.00%   +0.14%     
==========================================
  Files          25       25              
  Lines        2792     2792              
==========================================
+ Hits         2481     2485       +4     
+ Misses        311      307       -4     

Impacted Files	Coverage Δ
src/parsing/proto_types.jl	91.25% <100.00%> (ø)
src/parsing/Parsers.jl	73.33% <0.00%> (+3.33%)	:arrow_up:

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