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/environ/src/module_environ.rs

@@ -3,15 +3,13 @@ use crate::module::{
     ModuleSignature, ModuleType, ModuleUpvar, TableInitializer, TablePlan, TypeTables,
 };
 use crate::tunables::Tunables;
-use cranelift_codegen::ir;
-use cranelift_codegen::isa::TargetFrontendConfig;
 use cranelift_codegen::packed_option::ReservedValue;
 use cranelift_entity::PrimaryMap;
 use cranelift_wasm::{
     self, translate_module, Alias, DataIndex, DefinedFuncIndex, ElemIndex, EntityIndex, EntityType,
     FuncIndex, Global, GlobalIndex, GlobalInit, InstanceIndex, InstanceTypeIndex, Memory,
-    MemoryIndex, ModuleIndex, ModuleTypeIndex, SignatureIndex, Table, TableIndex,
-    TargetEnvironment, TypeIndex, WasmError, WasmFuncType, WasmResult,
+    MemoryIndex, ModuleIndex, ModuleTypeIndex, SignatureIndex, Table, TableIndex, TypeIndex,
+    WasmError, WasmFuncType, WasmResult,
 };
 use std::collections::{hash_map::Entry, HashMap};
 use std::convert::TryFrom;
@@ -45,7 +43,6 @@ pub struct ModuleEnvironment<'data> {
 
     // Various bits and pieces of configuration
     features: WasmFeatures,
-    target_config: TargetFrontendConfig,
     tunables: Tunables,
     first_module: bool,
 }
@@ -134,18 +131,13 @@ pub struct FunctionMetadata {
 
 impl<'data> ModuleEnvironment<'data> {
     /// Allocates the environment data structures.
-    pub fn new(
-        target_config: TargetFrontendConfig,
-        tunables: &Tunables,
-        features: &WasmFeatures,
-    ) -> Self {
+    pub fn new(tunables: &Tunables, features: &WasmFeatures) -> Self {
         Self {
             result: ModuleTranslation::default(),
             results: Vec::with_capacity(1),
             in_progress: Vec::new(),
             modules_to_be: 1,
             types: Default::default(),
-            target_config,
             tunables: tunables.clone(),
             features: *features,
             first_module: true,
@@ -153,10 +145,6 @@ impl<'data> ModuleEnvironment<'data> {
         }
     }
 
-    fn pointer_type(&self) -> ir::Type {
-        self.target_config.pointer_type()
-    }
-
     /// Translate a wasm module using this environment.
     ///
     /// This consumes the `ModuleEnvironment` and produces a list of
@@ -378,16 +366,6 @@ impl<'data> ModuleEnvironment<'data> {
     }
 }
 
-impl<'data> TargetEnvironment for ModuleEnvironment<'data> {
-    fn target_config(&self) -> TargetFrontendConfig {
-        self.target_config
-    }
-
-    fn reference_type(&self, ty: cranelift_wasm::WasmType) -> ir::Type {
-        crate::reference_type(ty, self.pointer_type())
-    }
-}
-
 /// This trait is useful for `translate_module` because it tells how to translate
 /// environment-dependent wasm instructions. These functions should not be called by the user.
 impl<'data> cranelift_wasm::ModuleEnvironment<'data> for ModuleEnvironment<'data> {