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.

Cargo.toml

@@ -74,12 +74,7 @@ members = [
 
 [features]
 default = ["jitdump", "wasmtime/wat", "wasmtime/parallel-compilation"]
-lightbeam = [
-    "wasmtime-environ/lightbeam",
-    "wasmtime-jit/lightbeam",
-    "wasmtime-wast/lightbeam",
-    "wasmtime/lightbeam",
-]
+lightbeam = ["wasmtime/lightbeam"]
 jitdump = ["wasmtime/jitdump"]
 vtune = ["wasmtime/vtune"]