Avoid indexing and use iteration (#98)

...while still appeasing the borrow checker by taking spillsets out of `self`
and then putting them back in again when we're done.

I was just doing this to enable playing with some data structures in follow up
commits, but decided to benchmark this commit as-is and found 2-4% speed ups to
Cranelift compilation!

```
compilation :: instructions-retired :: benchmarks/bz2/benchmark.wasm

  Δ = 39946528.13 ± 38398.29 (confidence = 99%)

  no-index.so is 1.04x to 1.04x faster than main.so!

  [985704952 985984130.24 986180413] main.so
  [945649144 946037602.11 946262076] no-index.so

compilation :: instructions-retired :: benchmarks/pulldown-cmark/benchmark.wasm

  Δ = 48413802.56 ± 34288.05 (confidence = 99%)

  no-index.so is 1.03x to 1.03x faster than main.so!

  [1593663899 1593926801.92 1594246604] main.so
  [1545196678 1545512999.36 1545802144] no-index.so

compilation :: instructions-retired :: benchmarks/spidermonkey/benchmark.wasm

  Δ = 841028066.56 ± 253404.59 (confidence = 99%)

  no-index.so is 1.02x to 1.02x faster than main.so!

  [34798712681 34801346430.28 34802786661] main.so
  [33958847844 33960318363.72 33962177143] no-index.so
```

src/ion/spill.rs

@@ -89,9 +89,11 @@ impl<'a, F: Function> Env<'a, F> {
         spillslot: SpillSlotIndex,
     ) {
         self.spillsets[spillset.index()].slot = spillslot;
-        for i in 0..self.spillsets[spillset.index()].vregs.len() {
-            // don't borrow self
-            let vreg = self.spillsets[spillset.index()].vregs[i];
+
+        // Take `spillsets` to avoid a conflicting borrow of `self`.
+        let spillsets = std::mem::take(&mut self.spillsets);
+
+        for vreg in &spillsets[spillset.index()].vregs {
             trace!(
                 "spillslot {:?} alloc'ed to spillset {:?}: vreg {:?}",
                 spillslot,
@@ -112,6 +114,10 @@ impl<'a, F: Function> Env<'a, F> {
                     .insert(LiveRangeKey::from_range(&entry.range), entry.index);
             }
         }
+
+        // Replace `spillsets`.
+        let default = std::mem::replace(&mut self.spillsets, spillsets);
+        debug_assert!(default.is_empty());
     }
 
     pub fn allocate_spillslots(&mut self) {