Also declare MSRV in Cargo.toml

[glts]

Follow-up to #128: Also declare the MSRV in Cargo.toml, for the benefit of dependent apps and libraries.

See https://doc.rust-lang.org/cargo/reference/manifest.html#the-rust-version-field.

Thank you

[partim]

Thank you for the PR! As part of #142 I have to increase the Rust version to 1.59. Do you mind if I just introduce the field as part of that?

[glts]

@partim Of course, thank you

[partim]

Just to note it: The attribute was now added as part of #143.