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-6.wat

@@ -0,0 +1,11 @@
+(module
+  (func (export "foo")
+    i32.const 1
+    ;; Fewer params than results.
+    (block (param i32) (result i32 i64)
+      i64.const 2
+    )
+    drop
+    drop
+  )
+)