add merge_requirements method to CPAN::Meta::Prereqs

This addresses a pattern I find myself writing over and over -- collapsing a bunch of requirements down to a single set.  The default is "runtime, build, test" for "requires, recommends" because that is what CPAN clients should eventually be installing in the most common case.  (We hope.  Someday.)

@rjbs, please take a look before I merge this and ship it.