cranelift-wasm: Create ModuleTranslationState and polish API a little (#1111)

* cranelift-wasm: replace `WasmTypesMap` with `ModuleTranslationState`

The `ModuleTranslationState` contains information decoded from the Wasm module
that must be referenced during each Wasm function's translation.

This is only for data that is maintained by `cranelift-wasm` itself, as opposed
to being maintained by the embedder. Data that is maintained by the embedder is
represented with `ModuleEnvironment`.

A `ModuleTranslationState` is returned by `translate_module`, and can then be
used when translating functions from that module.

* cranelift-wasm: rename `TranslationState` to `FuncTranslationState`

To disambiguate a bit with the new `ModuleTranslationState`.

* cranelift-wasm: Reorganize the internal `state` module into submodules

One module for the `ModuleTranslationState` and another for the
`FuncTranslationState`.

* cranelift-wasm: replace `FuncTranslator` with methods on `ModuleTranslationState`

`FuncTranslator` was two methods that always took ownership of `self`, so it
didn't really make sense as an object as opposed to two different functions, or
in this case methods on the object that actually persists for a longer time.

I think this improves ergonomics nicely.

Before:

```rust
let module_translation = translate_module(...)?;
for body in func_bodies {
    let mut translator = FuncTranslator::new();
    translator.translate(body, ...)?;
}
```

After:

```rust
let module_translation = translate_module(...)?;
for body in func_bodies {
    module_translation.translate_func(body, ...)?;
}
```

Note that this commit does not remove `FuncTranslator`. It still exists, but is
just a wrapper over the `ModuleTranslationState` methods, and it is marked
deprecated, so that downstream users get a heads up. This should make the
transition easier.

* Revert "cranelift-wasm: replace `FuncTranslator` with methods on `ModuleTranslationState`"

This reverts commit 075f9ae933bcaae39348b61287c8f78a4009340d.

cranelift/wasm/src/state/module_state.rs

@@ -0,0 +1,27 @@
+use crate::translation_utils::SignatureIndex;
+use cranelift_entity::PrimaryMap;
+use std::boxed::Box;
+
+/// Contains information decoded from the Wasm module that must be referenced
+/// during each Wasm function's translation.
+///
+/// This is only for data that is maintained by `cranelift-wasm` itself, as
+/// opposed to being maintained by the embedder. Data that is maintained by the
+/// embedder is represented with `ModuleEnvironment`.
+#[derive(Debug)]
+pub struct ModuleTranslationState {
+    /// A map containing a Wasm module's original, raw signatures.
+    ///
+    /// This is used for translating multi-value Wasm blocks inside functions,
+    /// which are encoded to refer to their type signature via index.
+    pub(crate) wasm_types:
+        PrimaryMap<SignatureIndex, (Box<[wasmparser::Type]>, Box<[wasmparser::Type]>)>,
+}
+
+impl ModuleTranslationState {
+    pub(crate) fn new() -> Self {
+        ModuleTranslationState {
+            wasm_types: PrimaryMap::new(),
+        }
+    }
+}