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/fac-multi-value.wat

@@ -0,0 +1,19 @@
+(module
+  ;; Iterative factorial without locals.
+  (func $pick0 (param i64) (result i64 i64)
+    (get_local 0) (get_local 0)
+  )
+  (func $pick1 (param i64 i64) (result i64 i64 i64)
+    (get_local 0) (get_local 1) (get_local 0)
+  )
+  (func (export "fac-ssa") (param i64) (result i64)
+    (i64.const 1) (get_local 0)
+    (loop $l (param i64 i64) (result i64)
+      (call $pick1) (call $pick1) (i64.mul)
+      (call $pick1) (i64.const 1) (i64.sub)
+      (call $pick0) (i64.const 0) (i64.gt_u)
+      (br_if $l)
+      (drop) (return)
+    )
+  )
+)