Updating the MSRV to Rust 1.56

[sunfishcode]

PR #91 proposes to update errno's Minimum Supported Rust Version (MSRV) from 1.48 to 1.56. I plan to release this in errno 0.3.8, without a semver bump. I'm raising this in a new issue to give it some extra visibility and an opportunity for feedback.

1.56 is still quite conservative; it's older than the MSRV of many popular crates in the Rust ecosystem.

This is a tradeoff, and one which has seen much discussion in broader venues, but in this instance specifically: a semver bump would create significant hassles for a large number of users, while not making a semver bump should be only a relatively minor inconvenience—pinning to errno 0.3.7—for what I expect is a relatively small number of users.

[sunfishcode]

Ok, I've now merged #91.