refactor(form_mapping.go): mapping ptr, struct and map

[vkd]

• fix BindQuery not support time pointer, when non specified form fieldName #1672 correct work with ptr - not create value if field is not set
• avoid allocations on strings.Split() - change to strings.Index()
• fix Binding tag: add 'skipped' on top of 'required' #610 tag value "-" is mean ignoring field
• struct fields mapped like json.Unmarshal
• map fields mapped like json.Unmarshal

Run benchmark:
go test -run=NONE -benchmem -bench=BenchmarkMapForm ./binding -benchtime=20s > new.bench

Compare benchcmp old.bench new.bench:

benchmark                  old ns/op     new ns/op     delta
BenchmarkMapFormFull-4     2706          2476          -8.50%
BenchmarkMapFormName-4     271           228           -15.87%

benchmark                  old allocs     new allocs     delta
BenchmarkMapFormFull-4     22             12             -45.45%
BenchmarkMapFormName-4     3              1              -66.67%

benchmark                  old bytes     new bytes     delta
BenchmarkMapFormFull-4     384           200           -47.92%
BenchmarkMapFormName-4     40            8             -80.00%

[codecov]

Codecov Report

Merging #1749 into master will increase coverage by 0.11%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1749      +/-   ##
==========================================
+ Coverage    98.5%   98.62%   +0.11%     
==========================================
  Files          41       41              
  Lines        2073     2103      +30     
==========================================
+ Hits         2042     2074      +32     
+ Misses         19       18       -1     
+ Partials       12       11       -1

Impacted Files	Coverage Δ
binding/form_mapping.go	100% <100%> (ø)	⬆️
binding/form.go	100% <0%> (+7.4%)	⬆️

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[thinkerou]

LGTM, thanks!

[thinkerou]

@appleboy please review the pull request, thanks!