Separate testing utilities into their own crate

[fjarri]

In the testing submodule we have some useful utilities. It would be nice to gate them behind a feature so that regular crate users didn't need to compile that part of the code. The problem is, we use those utilities in manul's own unit-tests, and it's currently impossible to enable a feature for tests only (see rust-lang/cargo#2911).

So a possible solution is to separate the testing utilities into another crate, and use it in dev-dependencies of manul.

[fjarri]

Gated under a feature in #31. Writing doctests may be awkward, but then again, we're better off putting such tests in example. Closing for now.