possible fix for multi-method import bug

This patch allows you to import multi methods from several modules.
It merges the dispachtee lists. Two identical dispatchee signatures
dont clash yet, since I am unable to compare signatures.
Merge branch 'nom' of github.com:rakudo/rakudo into froggs_mergemulti
throw X::Import::OnlystarProto

Only if both sides have an onlystar proto the merge is allowed.
added test file import-multi.t

These tests cover the import and merging of multi subs and traits.
Merge branch 'nom' of github.com:rakudo/rakudo into froggs_mergemulti
Merge branch 'nom' of github.com:rakudo/rakudo into froggs_mergemulti
Merge branch 'nom' into froggs_mergemulti
Simplify and make multi merging a bit more robust.

Better duplicate elimination, and more reliable determination of what
is/isn't a dispatcher. Passes all the same tests.
Merge branch 'froggs_mergemulti' into nom