Trace String.prototype.substring for two-arg case.
* Export str_substring as js_str_substring.
* Add basic String_p_substring builtin (only handle end > begin, both in range).
* Add String_p_substring_1 builtin for the missing-end case.
* INS_CONST for named constants in traces.
* Support boxing of strings.
* Support CALLPROP with primitive this.
* Support traceable natives which require cx and this.
* Support fallible traceable natives.
* Fix JSOP_LENGTH to use i2f on result (need that everything-is-doubles T-shirt).
* Add strings test.