peepmatic: Introduce the main peepmatic crate

Peepmatic is a DSL for peephole optimizations and compiler for generating
peephole optimizers from them. The user writes a set of optimizations in the
DSL, and then `peepmatic` compiles the set of optimizations into an efficient
peephole optimizer:

```
DSL ----peepmatic----> Peephole Optimizer
```

The generated peephole optimizer has all of its optimizations' left-hand sides
collapsed into a compact automata that makes matching candidate instruction
sequences fast.

The DSL's optimizations may be written by hand or discovered mechanically with a
superoptimizer like [Souper][]. Eventually, `peepmatic` should have a verifier
that ensures that the DSL's optimizations are sound, similar to what [Alive][]
does for LLVM optimizations.

[Souper]: https://github.com/google/souper
[Alive]: https://github.com/AliveToolkit/alive2

cranelift/peepmatic/examples/simple.peepmatic

@@ -0,0 +1,11 @@
+(=> (bor $x (bor $x $y))
+    (bor $x $y))
+
+(=> (bor $y (bor $x $y))
+    (bor $x $y))
+
+(=> (bor (bor $x $y) $x)
+    (bor $x $y))
+
+(=> (bor (bor $x $y) $y)
+    (bor $x $y))