Treat `any_args` as an arg splat.

This allows it to match an arbitrary number of
arguments at any point in an arg list.

Closes #707.
Convert stateless arg matchers to singletons.

This saves on memory (fewer objects to GC!).
Provide a clear failure when invalid args are passed to `with`.
Fix `with` verified double verification to work with new `any_args` semantics.
Update `any_args` docs to explain splat semantics.
Simplify wording based on @xaviershay’s suggestion.
Merge pull request #786 from rspec/any-args-splat

Treat `any_args` as an arg splat.