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/tests/wasm_testsuite.rs

@@ -31,6 +31,7 @@ fn testsuite() {
     let flags = Flags::new(settings::builder());
     for path in paths {
         let path = path.path();
+        println!("=== {} ===", path.display());
         let data = read_module(&path);
         handle_module(data, &flags, ReturnMode::NormalReturns);
     }