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`.

crates/runtime/src/instance/allocator/pooling.rs

@@ -1051,8 +1051,7 @@ mod test {
     use crate::{Imports, VMSharedSignatureIndex};
     use wasmtime_environ::{
         entity::EntityRef,
-        ir::Type,
-        wasm::{Global, GlobalInit, Memory, SignatureIndex, Table, TableElementType, WasmType},
+        wasm::{Global, GlobalInit, Memory, SignatureIndex, Table, WasmType},
         MemoryPlan, ModuleType, TablePlan, TableStyle,
     };
 
@@ -1088,7 +1087,6 @@ mod test {
             style: TableStyle::CallerChecksSignature,
             table: Table {
                 wasm_ty: WasmType::FuncRef,
-                ty: TableElementType::Func,
                 minimum: 0,
                 maximum: None,
             },
@@ -1144,7 +1142,6 @@ mod test {
 
         module.globals.push(Global {
             wasm_ty: WasmType::I32,
-            ty: Type::int(32).unwrap(),
             mutability: false,
             initializer: GlobalInit::I32Const(0),
         });
@@ -1208,7 +1205,6 @@ mod test {
             style: TableStyle::CallerChecksSignature,
             table: Table {
                 wasm_ty: WasmType::FuncRef,
-                ty: TableElementType::Func,
                 minimum: 0,
                 maximum: None,
             },
@@ -1258,7 +1254,6 @@ mod test {
 
         module.globals.push(Global {
             wasm_ty: WasmType::I32,
-            ty: Type::int(32).unwrap(),
             mutability: false,
             initializer: GlobalInit::I32Const(0),
         });
@@ -1281,7 +1276,6 @@ mod test {
             style: TableStyle::CallerChecksSignature,
             table: Table {
                 wasm_ty: WasmType::FuncRef,
-                ty: TableElementType::Func,
                 minimum: 11,
                 maximum: None,
             },