cranelift-wasm: support multi-value Wasm (#1049)

This commit introduces initial support for multi-value Wasm. Wasm blocks and
calls can now take and return an arbitrary number of values.

The encoding for multi-value blocks means that we need to keep the contents of
the "Types" section around when translating function bodies. To do this, we
introduce a `WasmTypesMap` type that maps the type indices to their parameters
and returns, construct it when parsing the "Types" section, and shepherd it
through a bunch of functions and methods when translating function bodies.

cranelift/wasm/src/lib.rs

@@ -59,7 +59,7 @@ mod translation_utils;
 
 pub use crate::environ::{
     DummyEnvironment, FuncEnvironment, GlobalVariable, ModuleEnvironment, ReturnMode, WasmError,
-    WasmResult,
+    WasmResult, WasmTypesMap,
 };
 pub use crate::func_translator::FuncTranslator;
 pub use crate::module_translator::translate_module;