[ci] Separately track MSRV and MWRV

[joshlf]

MWRV stands for "minimum working Rust version" (a term we invent here). It is the minimum version of the toolchain that we test with in CI. We maintain (and test) the invariant that MSRV >= MWRV so that we know at all times that our code is compatible with our published MSRV.

The reason for doing this is so that we can make progress towards lowering our MSRV without painting ourselves into a corner. We treat MSRV bumps as breaking changes, so if we were to publish a particular MSRV and then later discover a problem, we wouldn't be able to revert to a previously-published, higher MSRV. By tracking MWRV separately, we can ensure that we're making progress while still leaving ourselves wiggle room to revert changes so long as those reversions are compatible with our published MSRV. Once we have let a particular MWRV bake long enough, we can lower the published MSRV to match.

Closes #807

[joshlf]

@jswrenn I've confirmed locally that this doesn't have the same bug as #810.