Planned MSRV bump to Rust 1.51 near 2021-09-25

[jhpratt]

This is just under seven weeks away from the time of posting, so this should be more than sufficient notice. I intend on bumping the MSRV to Rust 1.51, which will have been released for six months on 2021-09-25. The bump will occur on or after this date.

The primary reason for the MSRV bump is to allow for use of min const generics. Initially this usage will be limited to internal optimization for the parser, though I could imagine a limited form of typestate being used to help enforce guarantees. The Cargo.toml for time will also be updated to use resolver= "2", which was included in the same Rust release. Other changes can also be made internally, though nothing notable.

No pre-existing public APIs will be affected by this MSRV bump.