This declares the minimum supported Rust version, independently of the rust-toolchain.toml file, which specifies the Rust version and toolchain components the crate/workspace should be built with. The MSRV affects other crates that depend on anything in this workspace, but the toolchain file does not seem to affect them. This means the MSRV is useful for those who build or develop dependents.

Signed-off-by: Lars Wirzenius liw@liw.fi