Distinguish between MSRV and minimum *working* Rust version

[jswrenn]

With #791, we have the opportunity to decrease our MSRV to as low as 1.58. For 0.8's initial release, we should not target an MSRV this low. Our current policy is to treat MSRV increases as a breaking changes. Since breaking changes should be infrequent, we should be careful not to back ourselves into a corner where our MSRV blocks us from providing vital functionality in a timely fashion.

Nonetheless, we should still strive for zerocopy to work on old as a Rust version as possible. To facilitate this, I propose that we distinguish between MSRV and Minimum Working Rust Version (MWRV). Our MSRV, encoded in the rust-version field of our Cargo.toml should remain at ~1.60 and decremented cautiously. Our MWRV, encoded package.metadata.ci in Cargo.toml should be as low as technically possible at all times. We should modify our CI to test on MWRV, instead of MSRV, and additionally check that MWRV <= MSRV.

[joshlf]

Reopening per #811.