flatc support

[rjkat]

• default values, syntax and macro-fu borrowed from Parameters.jl
• offsets were not always the same as the ones generated by the flatc compiler, so added a new slot_offsets() function
• hacks to workaround lack of support for forward type declarations in Julia
• limited tests for binary compatibility, more testing required

[codecov[bot]]

Codecov Report

Merging #33 into master will increase coverage by 11.31%. The diff coverage is 79.27%.

@@             Coverage Diff             @@
##           master      #33       +/-   ##
===========================================
+ Coverage   68.61%   79.93%   +11.31%     
===========================================
  Files           3        3               
  Lines         411      573      +162     
===========================================
+ Hits          282      458      +176     
+ Misses        129      115       -14

Impacted Files	Coverage Δ
src/macros.jl	77.77% <76.66%> (+3.62%)	:arrow_up:
src/FlatBuffers.jl	79.51% <78.37%> (+13.64%)	:arrow_up:
src/internals.jl	82.38% <87.69%> (+14%)	:arrow_up:

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[rjkat]

Looks like a lot of the coverage changes are bogus (in lines that I haven't touched). Not sure how to go about fixing this, will investigate.

[rjkat]

Okay! The code coverage reports on master were indeed bogus. This is ready to merge, pending your approval @quinnj