More closely match python splatting for calls

More closely match python splatting for calls

In Python, it's legal to use splats in function calls like this:

  call_function('123', *b, c=5, **d)

Prior to this change, jinja2 did not allow that. It mandated that all
splatting had to take place at the end of the call:

  call_function('123', c=5, *b, **d)

This commit allows both orders.

Additionally, this commit adds tests for more things that are illegal:
namely giving a function a positional argument after using a splat to
unpack positional arguments or giving a function a named argument after
using a double splat to unpack keyword arguments.