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-13.wat

@@ -0,0 +1,10 @@
+(module
+  (func (export "as-if-then") (param i32 i32) (result i32)
+    (block (result i32)
+      (if (result i32) (local.get 0)
+        (then (br 1 (i32.const 3)))
+        (else (local.get 1))
+      )
+    )
+  )
+)