* 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
The `vreg_def_blockparam` and `vreg_def_inst` fields on CFGInfo are only
used in `validate_ssa`, which in turn is only used in the ssagen fuzz
target.
Since these fields are never read in normal usage, initializing them is
entirely wasted effort. According to valgrind/DHAT, when running
`wasmtime compile` on the Sightglass Spidermonkey benchmark, removing
these fields saves about 100M instructions, 23k heap allocations
totalling 40MiB, and 47MiB of writes to the heap.
