refactor(form_mapping.go): mapping ptr, struct and map (#1749)

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

* fix #1672 correct work with ptr - not create value if field is not set
* avoid allocations on strings.Split() - change to strings.Index()
* fix #610 tag value "-" is mean ignoring field
* struct fields mapped like json.Unmarshal
* map fields mapped like json.Unmarshal

* fix after @thinkerou review

binding/form_mapping_benchmark_test.go

@@ -0,0 +1,61 @@
+// Copyright 2019 Gin Core Team.  All rights reserved.
+// Use of this source code is governed by a MIT style
+// license that can be found in the LICENSE file.
+
+package binding
+
+import (
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/assert"
+)
+
+var form = map[string][]string{
+	"name":      {"mike"},
+	"friends":   {"anna", "nicole"},
+	"id_number": {"12345678"},
+	"id_date":   {"2018-01-20"},
+}
+
+type structFull struct {
+	Name    string   `form:"name"`
+	Age     int      `form:"age,default=25"`
+	Friends []string `form:"friends"`
+	ID      *struct {
+		Number      string    `form:"id_number"`
+		DateOfIssue time.Time `form:"id_date" time_format:"2006-01-02" time_utc:"true"`
+	}
+	Nationality *string `form:"nationality"`
+}
+
+func BenchmarkMapFormFull(b *testing.B) {
+	var s structFull
+	for i := 0; i < b.N; i++ {
+		mapForm(&s, form)
+	}
+	b.StopTimer()
+
+	t := b
+	assert.Equal(t, "mike", s.Name)
+	assert.Equal(t, 25, s.Age)
+	assert.Equal(t, []string{"anna", "nicole"}, s.Friends)
+	assert.Equal(t, "12345678", s.ID.Number)
+	assert.Equal(t, time.Date(2018, 1, 20, 0, 0, 0, 0, time.UTC), s.ID.DateOfIssue)
+	assert.Nil(t, s.Nationality)
+}
+
+type structName struct {
+	Name string `form:"name"`
+}
+
+func BenchmarkMapFormName(b *testing.B) {
+	var s structName
+	for i := 0; i < b.N; i++ {
+		mapForm(&s, form)
+	}
+	b.StopTimer()
+
+	t := b
+	assert.Equal(t, "mike", s.Name)
+}