Check for byte buffer on decode

[hannahhoward]

See slack message about mem allocations and io.ReadAll

[codecov]

Codecov Report

Merging #3 into master will increase coverage by 0.98%.
The diff coverage is 84.62%.

@@            Coverage Diff             @@
##           master       #3      +/-   ##
==========================================
+ Coverage   22.24%   23.22%   +0.98%     
==========================================
  Files           7        7              
  Lines         787      797      +10     
==========================================
+ Hits          175      185      +10     
  Misses        596      596              
  Partials       16       16              

Impacted Files	Coverage Δ
coding.go	82.72% <84.62%> (+2.44%)	⬆️

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[rvagg]

might be a good chance to get @mvdan in here to help with reviews

[mvdan]

SGTM, though I'm not sure why these funcs take an io.Reader if we just grab the entire buffer all at once. This would be much more straightforward and performant if it took a []byte directly.