Make regalloc2 #![no_std] (#119)

* Make regalloc2 `#![no_std]`

This crate doesn't require any features from the standard library, so it
can be made `no_std` to allow it to be used in environments that can't
use the Rust standard library.

This PR mainly performs the following mechanical changes:
- `std::collections` is replaced with `alloc::collections`.
- `std::*` is replaced with `core::*`.
- `Vec`, `vec!`, `format!` and `ToString` are imported when needed since
  they are no longer in the prelude.
- `HashSet` and `HashMap` are taken from the `hashbrown` crate, which is
  the same implementation that the standard library uses.
- `FxHashSet` and `FxHashMap` are typedefs in `lib.rs` that are based on
  the `hashbrown` types.

The only functional change is that `RegAllocError` no longer implements
the `Error` trait since that is not available in `core`.

Dependencies were adjusted to not require `std` and this is tested in CI
by building against the `thumbv6m-none-eabi` target that doesn't have
`std`.

* Add the Error trait impl back under a "std" feature

.github/workflows/rust.yml

@@ -37,6 +37,16 @@ jobs:
     - name: Check with all features
       run: cargo check --all-features
 
+  # Make sure the code and its dependencies compile without std.
+  no_std:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v3
+    - name: Install thumbv6m-none-eabi target
+      run: rustup target add thumbv6m-none-eabi
+    - name: Check no_std build
+      run: cargo check --target thumbv6m-none-eabi --no-default-features --features trace-log,checker,enable-serde
+
   # Lint dependency graph for security advisories, duplicate versions, and
   # incompatible licences.
   cargo_deny: