Fix #470: Prevent error AsyncToSync.main_wrap() got multiple values for argument '<kwarg>'

[Krismix1]

Fixes #470.

Destructuring *args and **kwargs passed in by the callee has the chance that the same kwargs names may be used as by main_wrap. If that happens, the interpreter will raise a error like: TypeError: AsyncToSync.main_wrap() got multiple values for argument 'context'.
ParamSpec requires the *args and **kwargs syntax to be used, but it is wrong execution wise.
So we work around the issue by pre-creating the awaitable outside main_wrap, which removes the need to pass *args and **kwargs.
Another alternative instead of creating the awaitable could be to use functools.partial, though that does add some performance overhead: #470 (comment).

[andrewgodwin]

That's a very neat way of solving it - nice! And thanks!