reflect: implement StructOf

This change exposes a facility to create new struct types from a slice of
reflect.StructFields.

- reflect: first stab at implementing StructOf
- reflect: tests for StructOf

Creating struct types with embedded interfaces with unexported methods
is not supported yet and will panic.

Binaries' sizes for linux_amd64:

old=tip (0104a31)

            old bytes     new bytes     delta
bin/go      9911336       9915456       +0.04%
reflect     781704        830048        +6.18%

Updates golang#5748.

Change-Id: I3b8fd4fadd6ce3b1b922e284f0ae72a3a8e3ce44

Author: sbinet

src/cmd/compile/internal/gc/reflect.go

@@ -1302,7 +1302,7 @@ ok:
 	// we want be able to find.
 	if t.Sym == nil {
 		switch t.Etype {
-		case TPTR32, TPTR64, TARRAY, TCHAN, TFUNC, TMAP:
+		case TPTR32, TPTR64, TARRAY, TCHAN, TFUNC, TMAP, TSTRUCT:
 			slink := typelinksym(t)
 			dsymptr(slink, 0, s, 0)
 			ggloblsym(slink, int32(Widthptr), int16(dupok|obj.RODATA))