Some combos of *args/**kwds in caller/callee not supported

[gvanrossum]

This spawned off #1553. The mapping of argument type from a call site to a function definition is complicated by the existence of *args, **kwds, optional args, and keyword-only args.

For example, Python 3.5+ support multiple *args in a call! (But not in a definition.) So this (valid) code gives two errors while there should be none:

def f(*a: int) -> int: return sum(a)
f(*[1, 2, 3], *[4, 5], 6)  # E: Too many arguments for "f"
f(*(1, 2, 3), *(4, 5), 6)  # E: Too many arguments for "f"

(It also doesn't give a SyntaxError with --python-version=3.4 as it should.)

[gvanrossum]

Here's another example of something that doesn't work:

def f(*a: int) -> None:
    print(a)

f(1, 2, 3, 4)  # ok
f(1, 2, *[3, 4])  # ok
f(*[1, 2], 3, 4)  # E: Too many arguments for "f"

The reason in this case is that [1, 2] is treated as just List[int] and a *x argument where x is a sequence type, when the corresponding formal is also a * argument, is assumed to "fill up" the entire *args.