Fix Rust toolchain version enforcement in GitHub Actions

[ebkalderon]

Added

• Add rust-toolchain file to enforce the correct Rust version when working locally.

Changed

• Bump project-wide minimum supported rust version (MSRV) to 1.59.0, matching the MSRV of dashmap.
• Enforce rust-version = "1.59.0" in tower-lsp crate.
• Enforce rust-version = "1.56.0" in tower-lsp-macros crate.
• Revert use of #[derive(Default)] on jsonrpc::Id enum (introduced in #368).
• Use actions/cache@v3 to cache per-(OS, version) toolchains (see https://github.com/actions-rs/toolchain/issues/54#issuecomment-1452937296).
  • I've decided to switch away from actions-rs to dtolnay/rust-toolchain instead. It's much faster, even without caching.

Fixed

• Fix Rust toolchain version enforcement in GitHub Actions.

Fixes #311.