Add bound_arg to signature() and Signature.from_callable
Have 2 more functions: getfullargspec_from_signature, getcallargs_from_fullargspec
Use those 2 functions in getfullargspec and getcallargs.
This way:
a) signatures of existing functions are (almost) not modified
b) Allows user to get what he needs with all parameter combinations
The only tiny ambiguous nuance that I can see is getcallargs implicitly retrieving __self__ if ismethod(func). If fullargspec was run with skip_bound_arg=True, then this step would need to be omitted.