Fix bugs with kwonly arguments

If a named argument comes after the *args it can only be specified by keyword. These hare handled differently inside python than normal positional arguments. A few bugs:

1) kwonly arguments did not have their default value checked.
2) the annotation on any kwonly argument would be improperly applied to the varargs in the function (which are allowed to be a different type)
3) all variables in the function are checked during decoration time, only the ones which are arguments need to be.

Unit tests added for cases 1 and 2. Not for 3 because it is purely performance with no visible change to output so it can't be tested traditionally.
I'm curious, does inlining this function give a significant performance gain?