Add some more overloading tests
First pass at proper overloading support

This still could use some more tests, but the role application bug is fixed,
and all other role-related tests seem to be passing, so yay.
Add/update basic tests for CMOP::Overload class
Make sure that overloads are cloned properly during role composition
Update docs for overloading APIs
Die if an exception class has no metaclass
Add Changes & Delta entries for overloading changes
Merge remote-tracking branch 'pulls/83' - Rewrite overloading implementation

This more or less matches my original proposed API at
https://gist.github.com/autarch/de572363115de4239b3a

It fixes the bug that broke HTTP-Throwable
(https://rt.cpan.org/Ticket/Display.html?id=98531) and generally makes
overloading support correct, including complete support for applying
overloading during role composition.

(Closes #83.)
declare new prereq
spelling fix
fix pod coverage test
bump version to 2.1306
remove unused imports
switched some uses of List::MoreUtils APIs to their List::Util equivalents
moved tests into xt/author that are expected to pass after every commit