adds support for accepting any value in mocked functions

[mcfunley]

Not sure if you'll like this or not, but here's support for mocking with "any" placeholders in argument slots. e.g.

(mock! #'foo {[:x :y] 1 [:z :mocko.core/any] 2})

The any keyword is namespaced to avoid mass hysteria. This required adding an assertion to mock! to make sure that you're not accidentally creating ambiguous cases, e.g.

;; both of these cases match, so this throws an exception
(mock! #'foo {[:x :y] 1 [:x :mocko.core/any] 2})