wasmtime: Extract cranelift/lightbeam compilers to separate crates (#2117)

This commit extracts the two implementations of `Compiler` into two
separate crates, `wasmtime-cranelfit` and `wasmtime-lightbeam`. The
`wasmtime-jit` crate then depends on these two and instantiates them
appropriately. The goal here is to start reducing the weight of the
`wasmtime-environ` crate, which currently serves as a common set of
types between all `wasmtime-*` crates. Long-term I'd like to remove the
dependency on Cranelift from `wasmtime-environ`, but that's going to
take a lot more work.

In the meantime I figure it's a good way to get started by separating
out the lightbeam/cranelift function compilers from the
`wasmtime-environ` crate. We can continue to iterate on moving things
out in the future, too.

crates/jit/src/compiler.rs

@@ -50,10 +50,10 @@ impl Compiler {
             strategy,
             compiler: match strategy {
                 CompilationStrategy::Auto | CompilationStrategy::Cranelift => {
-                    Box::new(wasmtime_environ::cranelift::Cranelift::default())
+                    Box::new(wasmtime_cranelift::Cranelift::default())
                 }
                 #[cfg(feature = "lightbeam")]
-                CompilationStrategy::Lightbeam => Box::new(wasmtime_environ::lightbeam::Lightbeam),
+                CompilationStrategy::Lightbeam => Box::new(wasmtime_lightbeam::Lightbeam),
             },
             tunables,
         }