Add compatibility with 1.34

[jyn514]

This PR is best reviewed commit-by-commit. See individual commits for a rationale of the changes.

[jyn514]

I want to add 1.34 to CI but I don't know how to use github actions.

[jyn514]

It looks like this doesn't actually run 1.34 and just downloads it temporarily before switching the default to nightly.

[Kixiron]

I think the best thing to do is either add a separate msrv job that just runs check on 1.34 or to use cargo-msrv in another job

[Kixiron]

Thank you for contributing! Cleaning up the code is much appreciated (and much needed), it'll be good to go after the CI issues get ironed out (Publishing a release may take a week or so though)

I think this should work for testing against 1.34.0

msrv:
  name: MSRV
  runs-on: ubuntu-latest

  strategy:
    fail-fast: false

  env:
    RUST_BACKTRACE: 1
    CARGO_INCREMENTAL: 0

  steps:
    - uses: actions/checkout@v2
    - uses: actions-rs/toolchain@v1
      with:
        profile: minimal
        toolchain: 1.34.0
        override: true
  
    - name: Check
      uses: actions-rs/cargo@v1
      with:
        command: check
        args: --features multi-threaded
    
    - name: Test
      uses: actions-rs/cargo@v1
      with:
        command: test
        args: --features multi-threaded

[jyn514]

Testing won't work because criterion isn't compatible with 1.34. The rest is super helpful though!

[jyn514]

--features multi-threaded also won't work because dashmap isn't compatible with 1.34.

error[E0658]: use of unstable library feature 'map_get_key_value' (see issue #49347)
   --> /home/joshua/.local/lib/cargo/registry/src/github.com-1ecc6299db9ec823/dashmap-3.11.5/src/lib.rs:695:37
    |
695 |         if let Some((k, v)) = shard.get_key_value(key) {
    |                                     ^^^^^^^^^^^^^

[Kixiron]

Thanks!