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/libcalls.rs

@@ -58,16 +58,14 @@
 
 use crate::externref::VMExternRef;
 use crate::instance::Instance;
-use crate::table::Table;
+use crate::table::{Table, TableElementType};
 use crate::traphandlers::{raise_lib_trap, Trap};
 use crate::vmcontext::{VMCallerCheckedAnyfunc, VMContext};
 use backtrace::Backtrace;
 use std::mem;
 use std::ptr::{self, NonNull};
 use wasmtime_environ::ir::TrapCode;
-use wasmtime_environ::wasm::{
-    DataIndex, ElemIndex, GlobalIndex, MemoryIndex, TableElementType, TableIndex,
-};
+use wasmtime_environ::wasm::{DataIndex, ElemIndex, GlobalIndex, MemoryIndex, TableIndex};
 
 const TOINT_32: f32 = 1.0 / f32::EPSILON;
 const TOINT_64: f64 = 1.0 / f64::EPSILON;
@@ -214,9 +212,7 @@ pub unsafe extern "C" fn wasmtime_table_grow(
     let table_index = TableIndex::from_u32(table_index);
     let element = match instance.table_element_type(table_index) {
         TableElementType::Func => (init_value as *mut VMCallerCheckedAnyfunc).into(),
-        TableElementType::Val(ty) => {
-            debug_assert_eq!(ty, crate::ref_type());
-
+        TableElementType::Extern => {
             let init_value = if init_value.is_null() {
                 None
             } else {
@@ -249,8 +245,7 @@ pub unsafe extern "C" fn wasmtime_table_fill(
                 let val = val as *mut VMCallerCheckedAnyfunc;
                 table.fill(dst, val.into(), len)
             }
-            TableElementType::Val(ty) => {
-                debug_assert_eq!(ty, crate::ref_type());
+            TableElementType::Extern => {
                 let val = if val.is_null() {
                     None
                 } else {