Garbage collect (some) strings.

Separate literal/interned and garbage collected string types in js_Value.

js_pushintern/js_tointern are convenience functions to push/pop strings and
automatically intern them (so that the string pointers are guaranteed to be
stable).

js_pushliteral should push stable strings (either interned or actual literals).

js_pushstring will copy the string into garbage collected memory.

The pointer returned by js_tostring is guaranteed to be stable only for as long as
the stack slot it came from remains untouched.

Some uses will always cause a string to be interned:

 * Using it as a property name.
 * Wrapping it in a new String() object.
 * Strings returned by toString().
	ToPrimitive must not clobber the stack, so the result has to be unrooted.
 * Numbers converted to strings (by js_tostring)
	Likewise, we have nowhere to store the temporary string here.
	Passing in a scratch buffer to js_tostring could help this problem.
	Mostly an issue with array accesses (OP_GETPROP, etc) so an auxiliary
	function and we don't have to clutter the API needlessly.

jsstring.c

@@ -55,7 +55,7 @@ static void jsB_new_String(js_State *J)
 
 static void jsB_String(js_State *J)
 {
-	js_pushliteral(J, js_gettop(J) > 1 ? js_tostring(J, 1) : "");
+	js_pushstring(J, js_gettop(J) > 1 ? js_tostring(J, 1) : "");
 }
 
 static void Sp_toString(js_State *J)