Memoize expressions in basic blocks

[wilcoxjay]

Quite a bit of refactoring along the way. This PR is a draft until egraphs-good/egglog#207, egraphs-good/egglog#213, and egraphs-good/egglog#214 are merged.

Until then, it's not easy to test this locally, but here's the punchline:

pub(crate) fn term_to_type(&self, id: &TermId) -> Type {
modified   tests/snapshots/files__add.snap
@@ -10,8 +10,7 @@ expression: "format!(\"{}\", res)"
   v0.0: int = const 1;
   v1.0: int = const 2;
   v2.0: int = const 3;
-  v0_: int = const 3;
-  print v0_;
+  print v2.0;
   ret;
 .sblock___1:
 .exit___:
modified   tests/snapshots/files__add_no_opt.snap
@@ -9,13 +9,8 @@ expression: "format!(\"{}\", res)"
 .b1:
   v0.0: int = const 1;
   v1.0: int = const 2;
-  v0_: int = const 1;
-  v1_: int = const 2;
-  v2.0: int = add v0_ v1_;
-  v3_: int = const 1;
-  v4_: int = const 2;
-  v2_: int = add v3_ v4_;
-  print v2_;
+  v2.0: int = add v0.0 v1.0;
+  print v2.0;
   ret;
 .sblock___1:
 .exit___:

We memoize!

[wilcoxjay]

The CI failures are expected and are due to the fact that this branch still points to egglog main. I will update the pointer when the underlying PRs merge.

[wilcoxjay]

The pointer is updated, and this is no longer a draft. Just waiting on CI fixes from @oflatt to rebase again.

[wilcoxjay]

Ok, this is successfully rebased and ready for review.

[oflatt]

Great job! Yayyyyy we can merge this!