Generate shell completions and man page for `b3sum` at build time

[sorairolake]

With this change, the following files will be generated when building b3sum:

• A man page (b3sum.1).
• Shell completions files for Bash (b3sum.bash), Elvish (b3sum.elv), fish (b3sum.fish), PowerShell (_b3sum.ps1), and Zsh (_b3sum).

To find where these files are generated run the following command in b3sum/:

find ./target -path '*/b3sum-*/out' -type d

To use these, install them to the destination (e.g., /usr/share/man/man1 or /usr/share/bash-completion/completions).

Also, I set rust-version to Cargo.toml. This is based on the value of .github/workflows/ci.yml. If there is no value, 1.56.0 is set based on edition.

This closes #190 and closes #235.

[sorairolake]

Bump the MSRV of blake3 to 1.72.0. This is because the MSRV of zeroize v1.8.0 is 1.72.0, and the CI will fail if the MSRV of blake3 isn't bumped.