Add a PermissiveRespondTo extension for Mashes

By default, Mashes don't state that they respond to unset keys. This
causes unexpected behavior when you try to use a Mash with a
SimpleDelegator.

This new extension allows you create a permissive subclass of Mash that
will be fully compatible with SimpleDelegator and allow you to fully do
thunk-oriented programming with Mashes.

This comes with the trade-off of a ~19KB cache for each of these
subclasses and a ~20% performance penalty on any of those subclasses.

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