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

README.md

@@ -1836,24 +1836,6 @@ $ curl "http://localhost:8080/getd?field_x=hello&field_d=world"
 {"d":"world","x":{"FieldX":"hello"}}
 ```
 
-**NOTE**: NOT support the follow style struct:
-
-```go
-type StructX struct {
-    X struct {} `form:"name_x"` // HERE have form
-}
-
-type StructY struct {
-    Y StructX `form:"name_y"` // HERE have form
-}
-
-type StructZ struct {
-    Z *StructZ `form:"name_z"` // HERE have form
-}
-```
-
-In a word, only support nested custom struct which have no `form` now.
-
 ### Try to bind body into different structs
 
 The normal methods for binding request body consumes `c.Request.Body` and they