Variadic templated `Function` (instead of only allowing 0-4 params)

[danluu]

This was originally implemented the way it's done in the existing code because variadic templates weren't available in VC++ when the project was started, and on Windows the first four params are passed as register, so adding more than four params is a new code path.

VC++ is now at least semi-modern so this should be possible now.