crate tests do not compile with MSRV 1.56 due to dependencies dropping support for that version

While bitvec compiles with rust 1.56, it looks like `crossbeam` by way of the `criterion` dependency no longer supports it:

```
% rustup run 1.56.1 cargo test
error: package `crossbeam-epoch v0.9.18` cannot be built because it requires rustc 1.61 or newer, while the currently active rustc version is 1.56.1
```

It appears the most recent version is 1.63.