Revisit the assertions libraries

[technosophos]

As discussed here, I think we're all having some mixed feelings about Spectral as the library we want to use for assertion testing. It can be a little verbose in lieu of simpler assertions (like the core assert!/assert_eq! macros). Now might be a good time to decide.

/cc @jlegrone and @glyn

[glyn]

There's a lot to be said for avoiding unnecessary dependencies, so I'd favour using core macros unless the code becomes a lot more verbose, in which case it might be worth adopting another library.