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/wasmtests/multi-0.wat

@@ -0,0 +1,3 @@
+(module
+  (func (export "i64.dup") (param i64) (result i64 i64)
+    (get_local 0) (get_local 0)))