Calling js_call with n < 0 led to us popping a negative number of items
from the stack, which could make us miss the stack size check.
Sanitize all uses of function.length in Function.prototype.apply and
Function.prototype.bind.
Revert 'add context and flag argument to js_newstate' commit.
The context argument just adds clutter. The flag which was intended
for JS_DEBUG and/or JS_STRICT shouldn't be necessary.
js_newcfunction and js_newcconstructor need an extra argument, the
name of the function to use in stack traces.
When a function does not use eval, arguments, with and has no inner
functions it does not need a closure. We can call it without creating
a variable object if we store local variables on the stack instead.
js_newuserdata(J, tag, ptr) creates a userdata object, wrapping the ptr.
It takes the prototype for the new object from the top of the stack, just
like js_newcconstructor does.
js_touserdata(J, tag, idx) extracts the userdata pointer. Throws a
TypeError if the object is not a userdata object with a matching tag.
js_isuserdata(J, tag, idx) checks if a value on the stack is a userdata
object with a matching tag.
