allow direct conversion between string and named []byte, []rune

The allowed conversions before and after are:
        type Tstring string
        type Tbyte []byte
        type Trune []rune

        string <-> string  // ok
        string <-> []byte  // ok
        string <-> []rune // ok
        string <-> Tstring // ok
        string <-> Tbyte // was illegal, now ok
        string <-> Trune // was illegal, now ok

        Tstring <-> string  // ok
        Tstring <-> []byte  // ok
        Tstring <-> []rune // ok
        Tstring <-> Tstring // ok
        Tstring <-> Tbyte // was illegal, now ok
        Tstring <-> Trune // was illegal, now ok

Update spec, compiler, tests.  Use in a few packages.

We agreed on this a few months ago but never implemented it.

Fixes #1707.

R=golang-dev, gri, r
CC=golang-dev
https://golang.org/cl/5421057

src/pkg/net/mail/message.go

@@ -185,7 +185,7 @@ func (a *Address) String() string {
 type addrParser []byte
 
 func newAddrParser(s string) *addrParser {
-	p := addrParser([]byte(s))
+	p := addrParser(s)
 	return &p
 }