Support binding for slice/array obj

[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)
}

[codecov]

Codecov Report

Merging #2194 into master will not change coverage by %.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2194   +/-   ##
=======================================
  Coverage   98.49%   98.49%           
=======================================
  Files          41       41           
  Lines        2319     2319           
=======================================
  Hits         2284     2284           
  Misses         20       20           
  Partials       15       15           

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