Macros to enhance the sigma-types crate

Universal type safety

The #[forall] macro automatically writes a test that ensures that, for any possible inputs, a function never panics (which would indicate a broken sigma-type guarantee). This helps enforce a consistent paradigm in which any "invariants" or special caveats ought to be readable directly off an item's type signature for its end-users: if a certain input combination causes a function to fail, then it ought to be explicitly disallowed at the type level. (Internally, this uses quicheck to do most of the heavy lifting.)