Add Clippy msrv configuration file

This allows us to automatically ignore lints that are
incompatible with our MSRV.

Signed-off-by: Joe Richey <joerichey@google.com>

.clippy.toml

@@ -0,0 +1 @@
+msrv = "1.34"

.github/workflows/tests.yml

@@ -174,8 +174,11 @@ jobs:
       - uses: actions-rs/toolchain@v1
         with:
           profile: minimal
-          toolchain: stable
+          # https://github.com/rust-lang/rust-clippy/pull/6379 added MSRV
+          # support, so we need to use nightly until this is on stable.
+          toolchain: nightly
           components: rustfmt, clippy
+          override: true
       - name: clippy
         run: cargo clippy --all
       - name: fmt

src/lib.rs

@@ -146,8 +146,6 @@
 )]
 #![no_std]
 #![warn(rust_2018_idioms, unused_lifetimes, missing_docs)]
-// `matches!` macro was added only in Rust 1.42, which is bigger than our MSRV
-#![allow(clippy::match_like_matches_macro)]
 
 #[macro_use]
 extern crate cfg_if;