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.
