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.

src/obj.rs

@@ -2,8 +2,6 @@ use anyhow::{bail, Context as _, Result};
 use object::write::Object;
 use target_lexicon::Triple;
 use wasmtime::Strategy;
-#[cfg(feature = "lightbeam")]
-use wasmtime_environ::Lightbeam;
 use wasmtime_environ::{settings, settings::Configurable, ModuleEnvironment, Tunables};
 use wasmtime_jit::{native, Compiler};