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/environ/mod.rs

@@ -7,4 +7,5 @@ mod spec;
 pub use crate::environ::dummy::DummyEnvironment;
 pub use crate::environ::spec::{
     FuncEnvironment, GlobalVariable, ModuleEnvironment, ReturnMode, WasmError, WasmResult,
+    WasmTypesMap,
 };