Make MXRP a metarole (aka trait) rather than a subclass of Moose::Meta::Role

This makes it play nice(r) with other MooseX modules.
Fix docs to match reality - see RT #93755
Replace parameters with arguments for clarity
Merge pull request #1 from moose/autarch/make-it-a-trait

Makes MXRP a metarole rather than a subclass of Moose::Meta::Role