Support binding for slice/array obj [Rewrite]

[wuhuizuo]

support usage followed:

type StructExample struct {
    A string `json:"a" form:"a" binding"required,max=16"`
}

func exampleHandler(c *gin.Context) {
    var data []StructExample

    if c.Bind(&data) != nil {
        return
    }
    c.JSON(http.StatusOK, data)
}

this pull is rewritten for https://github.com/gin-gonic/gin/pull/2194， avoid to change the validator interface

[wuhuizuo]

@thinkerou @appleboy I have rewritten for review issues in #2194

[codecov]

Codecov Report

Merging #2302 (d2db51d) into master (a573ec6) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2302      +/-   ##
==========================================
+ Coverage   98.47%   98.49%   +0.01%     
==========================================
  Files          41       41              
  Lines        1973     1989      +16     
==========================================
+ Hits         1943     1959      +16     
  Misses         17       17              
  Partials       13       13              

Impacted Files	Coverage Δ
binding/binding.go	100.00% <ø> (ø)
binding/default_validator.go	100.00% <100.00%> (ø)

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[appleboy]

@wuhuizuo Please fix the conflicts.

[wuhuizuo]

@wuhuizuo Please fix the conflicts.

done.

[appleboy]

@wuhuizuo build fail: https://travis-ci.org/github/gin-gonic/gin/builds/740820459

[thinkerou]

SYNTAX ERROR

[wuhuizuo]

sorry! it's fixed.

[thinkerou]

lgtm

[appleboy]

LGTM but build fail in go master version. See the report: https://travis-ci.org/github/gin-gonic/gin/jobs/742947069

[thinkerou]

@appleboy related issue: #2553