## Summary
Updates default toolchain to
[1.79](https://blog.rust-lang.org/2024/06/13/Rust-1.79.0.html).
There didn't seem to be any breaking changes besides xwin now requiring
lld llvm linker.
## Test Plan
Existing tests.
The `async fn` and return-position `impl Trait` in traits improve
`BuildContext` ergonomics. The traits use `impl Future` over `async fn`
to make the send bound explicit
(https://blog.rust-lang.org/2023/12/21/async-fn-rpit-in-traits.html).
The remaining changes are due to clippy.
