CIP-4127/CIP-4128: Gate Rust CI and declare MSRV

[tobyhede]

Summary

• gate CI on rustfmt, Clippy, and rustdoc warnings
• fix the six existing Clippy warnings
• declare Rust 1.94.1 as the MSRV
• test on both MSRV and stable Rust

Verification

• cargo fmt --all -- --check
• cargo clippy --all-targets --all-features -- -D warnings
• RUSTDOCFLAGS="-D warnings" cargo doc --all-features --no-deps
• cargo check --no-default-features
• cargo test on Rust 1.94.1 and stable

[CDThomas]

Looks like this is an older version of actions/checkout. Probably worth bumping actions to the latest versions while we're in the area and could also consider pinning to SHAs.

[freshtonic]

Approved with suggestion about not using such a recent rust-version in Cargo.toml.

[freshtonic]

The rust-version sets the minimum version of rustc that can successfully compile and run this package.

cargo will refuse to compile the package if rustc does not meet that minimum version requirement. And envelopers is a public package.

We should either set it to the minimum version of rustc that we can successfully compile the package with when --all-features is enabled or pick a version of rustc that is "old enough" to not exclude some TBC portion of rust users in the wild. 1.94.1 was release in March - it's still new-ish.