Remove dependency on TargetIsa from Wasmtime crates (#3178)

This commit started off by deleting the `cranelift_codegen::settings`
reexport in the `wasmtime-environ` crate and then basically played
whack-a-mole until everything compiled again. The main result of this is
that the `wasmtime-*` family of crates have generally less of a
dependency on the `TargetIsa` trait and type from Cranelift. While the
dependency isn't entirely severed yet this is at least a significant
start.

This commit is intended to be largely refactorings, no functional
changes are intended here. The refactorings are:

* A `CompilerBuilder` trait has been added to `wasmtime_environ` which
  server as an abstraction used to create compilers and configure them
  in a uniform fashion. The `wasmtime::Config` type now uses this
  instead of cranelift-specific settings. The `wasmtime-jit` crate
  exports the ability to create a compiler builder from a
  `CompilationStrategy`, which only works for Cranelift right now. In a
  cranelift-less build of Wasmtime this is expected to return a trait
  object that fails all requests to compile.

* The `Compiler` trait in the `wasmtime_environ` crate has been souped
  up with a number of methods that Wasmtime and other crates needed.

* The `wasmtime-debug` crate is now moved entirely behind the
  `wasmtime-cranelift` crate.

* The `wasmtime-cranelift` crate is now only depended on by the
  `wasmtime-jit` crate.

* Wasm types in `cranelift-wasm` no longer contain their IR type,
  instead they only contain the `WasmType`. This is required to get
  everything to align correctly but will also be required in a future
  refactoring where the types used by `cranelift-wasm` will be extracted
  to a separate crate.

* I moved around a fair bit of code in `wasmtime-cranelift`.

* Some gdb-specific jit-specific code has moved from `wasmtime-debug` to
  `wasmtime-jit`.

Cargo.lock

@@ -3564,7 +3564,6 @@ dependencies = [
  "bincode",
  "cfg-if 1.0.0",
  "cpp_demangle",
- "cranelift-native",
  "indexmap",
  "lazy_static",
  "libc",
@@ -3700,12 +3699,16 @@ dependencies = [
 name = "wasmtime-cranelift"
 version = "0.29.0"
 dependencies = [
+ "anyhow",
  "cranelift-codegen",
  "cranelift-entity",
  "cranelift-frontend",
+ "cranelift-native",
  "cranelift-wasm",
+ "gimli",
  "target-lexicon",
  "wasmparser",
+ "wasmtime-debug",
  "wasmtime-environ",
 ]
 
@@ -3727,6 +3730,7 @@ dependencies = [
 name = "wasmtime-environ"
 version = "0.29.0"
 dependencies = [
+ "anyhow",
  "cfg-if 1.0.0",
  "cranelift-codegen",
  "cranelift-entity",
@@ -3736,6 +3740,7 @@ dependencies = [
  "log",
  "more-asserts",
  "serde",
+ "target-lexicon",
  "thiserror",
  "wasmparser",
 ]
@@ -3806,7 +3811,6 @@ dependencies = [
  "thiserror",
  "wasmparser",
  "wasmtime-cranelift",
- "wasmtime-debug",
  "wasmtime-environ",
  "wasmtime-lightbeam",
  "wasmtime-obj",
@@ -3819,8 +3823,11 @@ dependencies = [
 name = "wasmtime-lightbeam"
 version = "0.29.0"
 dependencies = [
+ "anyhow",
  "cranelift-codegen",
+ "gimli",
  "lightbeam",
+ "target-lexicon",
  "wasmparser",
  "wasmtime-environ",
 ]
@@ -3833,7 +3840,6 @@ dependencies = [
  "more-asserts",
  "object",
  "target-lexicon",
- "wasmtime-debug",
  "wasmtime-environ",
 ]