ParseXS: refactor: IO_Param::lookup_output_typemap

Move all the code out of

ExtUtils::ParseXS::Node::IO_Param::as_output_code()

which is responsible for looking up the template output code in the
typemap (or elsewhere) and put it in it's own method,
lookup_output_typemap().

As well as splitting a 490-line method into two 200 and 340-line methods,
this will also allow us shortly to move the template lookup to earlier,
at parse time rather than code-emitting time.

It may also be possible at some point to merge the two methods added by
these last two commits, lookup_intput_typemap and lookup_output_typemap,
into a single method, since they share a lot of common code.

Author: iabyn