ParseXS: refactor: move sig parsing code into fn

Move the 200 or so lines of code which splits and parses an XSUB's
signature out from process_file() and into its own method.

This commit changes as little as possible: apart from adding a method
declaration and call, the main body of the code has just been cut and pasted
as-is, apart from reducing the indentation.

The next few commits will change and move the code so that it becomes
a method of ExtUtils::ParseXS::Node::Sig rather than of ExtUtils::ParseXS.

Author: iabyn