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

src/fuzzing/func.rs

@@ -8,6 +8,9 @@ use crate::{
     OperandConstraint, OperandKind, OperandPos, PReg, PRegSet, RegClass, VReg,
 };
 
+use alloc::vec::Vec;
+use alloc::{format, vec};
+
 use super::arbitrary::Result as ArbitraryResult;
 use super::arbitrary::{Arbitrary, Unstructured};
 
@@ -275,7 +278,7 @@ pub struct Options {
     pub reftypes: bool,
 }
 
-impl std::default::Default for Options {
+impl core::default::Default for Options {
     fn default() -> Self {
         Options {
             reused_inputs: false,
@@ -404,7 +407,7 @@ impl Func {
             }
             vregs_by_block.push(vregs.clone());
             vregs_by_block_to_be_defined.push(vec![]);
-            let mut max_block_params = u.int_in_range(0..=std::cmp::min(3, vregs.len() / 3))?;
+            let mut max_block_params = u.int_in_range(0..=core::cmp::min(3, vregs.len() / 3))?;
             for &vreg in &vregs {
                 if block > 0 && opts.block_params && bool::arbitrary(u)? && max_block_params > 0 {
                     block_params[block].push(vreg);
@@ -591,8 +594,8 @@ impl Func {
     }
 }
 
-impl std::fmt::Debug for Func {
-    fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
+impl core::fmt::Debug for Func {
+    fn fmt(&self, f: &mut core::fmt::Formatter) -> core::fmt::Result {
         write!(f, "{{\n")?;
         for vreg in self.reftype_vregs() {
             write!(f, "  REF: {}\n", vreg)?;
@@ -653,7 +656,7 @@ impl std::fmt::Debug for Func {
 }
 
 pub fn machine_env() -> MachineEnv {
-    fn regs(r: std::ops::Range<usize>) -> Vec<PReg> {
+    fn regs(r: core::ops::Range<usize>) -> Vec<PReg> {
         r.map(|i| PReg::new(i, RegClass::Int)).collect()
     }
     let preferred_regs_by_class: [Vec<PReg>; 2] = [regs(0..24), vec![]];