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

Cargo.toml

@@ -13,11 +13,15 @@ repository = "https://github.com/bytecodealliance/regalloc2"
 [dependencies]
 log = { version = "0.4.8", default-features = false }
 smallvec = { version = "1.6.1", features = ["union"] }
-fxhash = "0.2.1"
-slice-group-by = "0.3.0"
+rustc-hash = { version = "1.1.0", default-features = false }
+slice-group-by = { version = "0.3.0", default-features = false }
+hashbrown = "0.13.2"
 
 # Optional serde support, enabled by feature below.
-serde = { version = "1.0.136", features = ["derive"], optional = true }
+serde = { version = "1.0.136", features = [
+    "derive",
+    "alloc",
+], default-features = false, optional = true }
 
 # The below are only needed for fuzzing.
 libfuzzer-sys = { version = "0.4.2", optional = true }
@@ -29,7 +33,10 @@ debug-assertions = true
 overflow-checks = true
 
 [features]
-default = []
+default = ["std"]
+
+# Enables std-specific features such as the Error trait for RegAllocError.
+std = []
 
 # Enables generation of DefAlloc edits for the checker.
 checker = []