[WIP] Implement DecodeForm

[hypnoglow]

Ref: #19

[codecov]

Codecov Report

Merging #20 into master will decrease coverage by 2.34%.
The diff coverage is 81.1%.

@@            Coverage Diff             @@
##           master      #20      +/-   ##
==========================================
- Coverage   81.96%   79.61%   -2.35%     
==========================================
  Files          17       20       +3     
  Lines         571      623      +52     
==========================================
+ Hits          468      496      +28     
- Misses        100      112      +12     
- Partials        3       15      +12

Impacted Files	Coverage Δ
validate/validate.go	100% <ø> (ø)	⬆️
request_pathparam.go	100% <ø> (ø)	⬆️
validate/error.go	100% <100%> (ø)
decode_helpers.go	100% <100%> (ø)
decode_form.go	45% <45%> (ø)
decode_query.go	88.23% <88.23%> (ø)

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