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.
Add js_itoa for faster conversion of unsigned int to string.
Make js_toprimitive (and by consequence js_tonumber/string/...) in-place.

Any coercion between types may overwrite the stack slot with the coerced
value. This is usually not a problem, but if you need to preserve the
original value, you should copy it to another stack slot before running
any functions that may coerce the type (anything involving ToPrimitive).

This change lets us avoid interning the result of toString().

If we later add short strings (embedded in js_Value and js_Property
structs) then we can also avoid creating a garbage collected string or
interning the result of js_tostring on a number.
Add short strings (with data embedded in js_Value).

Allows js_tostring to avoid either interning strings converted from
numbers or creating lots of garbage collected strings.
Add user context pointer and flag argument to js_State constructor.
Silence GCC warning.