This cleans up the previous commit slightly by further reducing the
function call arguments to a single struct (containing the
function_record, arguments vector, and parent).

Although this doesn't currently change anything, it does allow for
future functionality to have a place for precalls to store temporary
objects that need to be destroyed after a function call (whether or not
the call succeeds).

As a concrete example, with this change #625 could be easily implemented
(I think) by adding a std::unique_ptr<gil_scoped_release> member to the
`function_call` struct with a precall that actually constructs it.
Without this, the precall can't do that: the postcall won't be invoked
if the call throws an exception.

This doesn't seems to affect the .so size noticeably (either way).