Support 4-byte UTF-8 sequences.

The following functions are no longer restricted to 16-bit integer values:

	String.fromCharCode()
	String.prototype.charCodeAt()

repr() will not escape SMP characters, as doing so would require conversion to
surrogate pairs, but will encode these characters as UTF-8. Unicode characters
in the BMP will still be escaped with \uXXXX as before.

JSON.stringify() only escapes control characters, so will represent all non-ASCII
characters as UTF-8.

We do no automatic conversions to/from surrogate pairs. Code that worked with
surrogate pairs should not be affected by these changes.

jsstring.c

@@ -310,7 +310,7 @@ static void S_fromCharCode(js_State *J)
 	}
 
 	for (i = 1; i < top; ++i) {
-		c = js_touint16(J, i);
+		c = js_touint32(J, i);
 		p += runetochar(p, &c);
 	}
 	*p = 0;