feat: improve get post data.

[appleboy]

benchmark program:

func BenchmarkGetPostFormArrayNew(b *testing.B) {
	c, _ := CreateTestContext(httptest.NewRecorder())
	c.Request = createMultipartRequest()

	var obj struct {
		Foo          string    `form:"foo"`
		Bar          string    `form:"bar"`
		BarAsInt     int       `form:"bar"`
		Array        []string  `form:"array"`
		ID           string    `form:"id"`
		TimeLocal    time.Time `form:"time_local" time_format:"02/01/2006 15:04"`
		TimeUTC      time.Time `form:"time_utc" time_format:"02/01/2006 15:04" time_utc:"1"`
		TimeLocation time.Time `form:"time_location" time_format:"02/01/2006 15:04" time_location:"Asia/Tokyo"`
		BlankTime    time.Time `form:"blank_time" time_format:"02/01/2006 15:04"`
	}
	c.Bind(&obj)

	for i := 0; i < b.N; i++ {
		c.GetPostFormArray("array")
	}
}

result:

$ go test -benchmem -run=^$ github.com/gin-gonic/gin -bench ^BenchmarkGetPostFormArray
goos: darwin
goarch: amd64
pkg: github.com/gin-gonic/gin
BenchmarkGetPostFormArrayOld-4          50000000                32.4 ns/op             0 B/op          0 allocs/op
BenchmarkGetPostFormArrayNew-4          50000000                22.3 ns/op             0 B/op          0 allocs/op
PASS
ok      github.com/gin-gonic/gin        2.847s

[codecov]

Codecov Report

Merging #1920 into master will increase coverage by <.01%.
The diff coverage is 80%.

@@            Coverage Diff             @@
##           master    #1920      +/-   ##
==========================================
+ Coverage   98.75%   98.75%   +<.01%     
==========================================
  Files          38       38              
  Lines        2167     2172       +5     
==========================================
+ Hits         2140     2145       +5     
  Misses         15       15              
  Partials       12       12

Impacted Files	Coverage Δ
context.go	98.42% <80%> (+0.01%)	⬆️

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