cmd/internal/gc: delete Strlit, Zconv

Strlit was just a poor excuse for a Go string.
Use a Go string.
In the one case where it was a string-or-nil (Type.Note), use a *string.

Zconv was a poor excuse for %q. Use %q.
The only important part about Zconv's implementation
was that the compiler and linker agreed on the quoting rules.
Now they both use %q instead of having two Zconvs.

This CL *does* change the generated object files, because the
quoted strings end up in symbol names.
For example the string "\r\n" used to be named go.string."\r\n"
and is now go.string."\x0d\n".

Change-Id: I5c0d38e1570ffc495f0db1a20273c9564104a7e8
Reviewed-on: https://go-review.googlesource.com/6519
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Rob Pike <r@golang.org>

src/cmd/internal/gc/sinit.go

@@ -443,7 +443,7 @@ func staticassign(l *Node, r *Node, out **NodeList) bool {
 	case OSTRARRAYBYTE:
 		if l.Class == PEXTERN && r.Left.Op == OLITERAL {
 			sval := r.Left.Val.U.Sval
-			slicebytes(l, sval.S, len(sval.S))
+			slicebytes(l, sval, len(sval))
 			return true
 		}
 
@@ -1368,7 +1368,7 @@ func iszero(n *Node) bool {
 			return true
 
 		case CTSTR:
-			return n.Val.U.Sval == nil || len(n.Val.U.Sval.S) == 0
+			return n.Val.U.Sval == ""
 
 		case CTBOOL:
 			return n.Val.U.Bval == 0