* rust-toolchain.toml: bump MSRV to 1.70.0
With Firefox 120 released on 21 November 2023, the MSRV is now 1.70.0.
* Fix cargo fmt with Rust 1.70.0
* Fix cargo clippy with Rust 1.70.0
* Fix cargo doc with Rust 1.70.0
* rust-toolchain.toml: add clippy component
* .github: bump dtolnay/rust-toolchain to 1.70
* helix-term: bump rust-version to 1.70
* helix-view/gutter: use checked_ilog10 to count digits
* helix-core/syntax: use MAIN_SEPARATOR_STR constant
* helix-view/handlers/dap: use Display impl for displaying process spawn error
* WIP: helix-term/commands: use checked math to assert ranges cannot overlap
This fixes breakage when installing `helix` due to an incorrect usage of
`as_ref()` when interacting with `bstr` in the `gitoxide` codebase.
However, this upgrade also requires a higher rustc version, as `gitoxide`
recently updated its `windows` crate version.
1.61.0 in particular introduced new clippy lints that unexpectedly
failed CI until addressed. The lints are a bit tough to fix since
the toolchain action starts using new rust versions almost immediately
after release, so if you aren't using rustup, you may have a hard
time reproducing the lint results until your package manager updates
rust.
This brings an extra burden that we have to remember to make a
commit/PR to update rust in CI.
