Make Function variadic and take a function signature.

Change-Id: If36ea6e74311f54bb4c2b0bc1b5b7ccd0e97d74b
Reviewed-on: https://swiftshader-review.googlesource.com/4548
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>
Tested-by: Nicolas Capens <capn@google.com>