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.rs

@@ -5,7 +5,7 @@
 use crate::export::Export;
 use crate::externref::VMExternRefActivationsTable;
 use crate::memory::{Memory, RuntimeMemoryCreator};
-use crate::table::{Table, TableElement};
+use crate::table::{Table, TableElement, TableElementType};
 use crate::traphandlers::Trap;
 use crate::vmcontext::{
     VMCallerCheckedAnyfunc, VMContext, VMFunctionImport, VMGlobalDefinition, VMGlobalImport,
@@ -25,7 +25,7 @@ use std::{mem, ptr, slice};
 use wasmtime_environ::entity::{packed_option::ReservedValue, EntityRef, EntitySet, PrimaryMap};
 use wasmtime_environ::wasm::{
     DataIndex, DefinedGlobalIndex, DefinedMemoryIndex, DefinedTableIndex, ElemIndex, EntityIndex,
-    FuncIndex, GlobalIndex, MemoryIndex, TableElementType, TableIndex, WasmType,
+    FuncIndex, GlobalIndex, MemoryIndex, TableIndex, WasmType,
 };
 use wasmtime_environ::{ir, HostPtr, Module, VMOffsets};
 
@@ -590,7 +590,7 @@ impl Instance {
                 )?;
             },
 
-            TableElementType::Val(_) => {
+            TableElementType::Extern => {
                 debug_assert!(elements.iter().all(|e| *e == FuncIndex::reserved_value()));
                 table.fill(dst, TableElement::ExternRef(None), len)?;
             }