encoding/xml: add, support Marshaler interface

See golang.org/s/go12xml for design.

Repeat of CL 12603044, which was submitted accidentally
and then rolled back.

Fixes golang#2771.
Fixes golang#4169.
Fixes golang#5975.
Fixes golang#6125.

R=golang-dev
CC=golang-dev
https://golang.org/cl/12919043

Author: rsc

api/go1.1.txt

@@ -92,7 +92,6 @@ pkg encoding/json, method (Number) String() string
 pkg encoding/json, type Number string
 pkg encoding/xml, func EscapeText(io.Writer, []uint8) error
 pkg encoding/xml, method (*Encoder) Indent(string, string)
-pkg encoding/xml, method (Encoder) ReadFrom(io.Reader) (int64, error)
 pkg encoding/xml, type Decoder struct, DefaultSpace string
 pkg go/ast, func NewCommentMap(*token.FileSet, Node, []*CommentGroup) CommentMap
 pkg go/ast, method (CommentMap) Comments() []*CommentGroup

api/go1.txt

@@ -2425,13 +2425,6 @@ pkg encoding/xml, method (*UnsupportedTypeError) Error() string
 pkg encoding/xml, method (CharData) Copy() CharData
 pkg encoding/xml, method (Comment) Copy() Comment
 pkg encoding/xml, method (Directive) Copy() Directive
-pkg encoding/xml, method (Encoder) Available() int
-pkg encoding/xml, method (Encoder) Buffered() int
-pkg encoding/xml, method (Encoder) Flush() error
-pkg encoding/xml, method (Encoder) Write([]uint8) (int, error)
-pkg encoding/xml, method (Encoder) WriteByte(uint8) error
-pkg encoding/xml, method (Encoder) WriteRune(int32) (int, error)
-pkg encoding/xml, method (Encoder) WriteString(string) (int, error)
 pkg encoding/xml, method (ProcInst) Copy() ProcInst
 pkg encoding/xml, method (StartElement) Copy() StartElement
 pkg encoding/xml, method (UnmarshalError) Error() string