peepmatic: Be generic over the operator type

This lets us avoid the cost of `cranelift_codegen::ir::Opcode` to
`peepmatic_runtime::Operator` conversion overhead, and paves the way for
allowing Peepmatic to support non-clif optimizations (e.g. vcode optimizations).

Rather than defining our own `peepmatic::Operator` type like we used to, now the
whole `peepmatic` crate is effectively generic over a `TOperator` type
parameter. For the Cranelift integration, we use `cranelift_codegen::ir::Opcode`
as the concrete type for our `TOperator` type parameter. For testing, we also
define a `TestOperator` type, so that we can test Peepmatic code without
building all of Cranelift, and we can keep them somewhat isolated from each
other.

The methods that `peepmatic::Operator` had are now translated into trait bounds
on the `TOperator` type. These traits need to be shared between all of
`peepmatic`, `peepmatic-runtime`, and `cranelift-codegen`'s Peepmatic
integration. Therefore, these new traits live in a new crate:
`peepmatic-traits`. This crate acts as a header file of sorts for shared
trait/type/macro definitions.

Additionally, the `peepmatic-runtime` crate no longer depends on the
`peepmatic-macro` procedural macro crate, which should lead to faster build
times for Cranelift when it is using pre-built peephole optimizers.

cranelift/peepmatic/crates/fuzzing/src/parser.rs

@@ -1,6 +1,7 @@
 //! Utilities for fuzzing our DSL's parser.
 
 use peepmatic::Optimizations;
+use peepmatic_test_operator::TestOperator;
 use std::str;
 
 /// Attempt to parse the given string as if it were a snippet of our DSL.
@@ -15,7 +16,7 @@ pub fn parse(data: &[u8]) {
         Err(_) => return,
     };
 
-    let _ = wast::parser::parse::<Optimizations>(&buf);
+    let _ = wast::parser::parse::<Optimizations<TestOperator>>(&buf);
 }
 
 #[cfg(test)]