Add initial support for the multi-memory proposal (#2263)

This commit adds initial (gated) support for the multi-memory wasm
proposal. This was actually quite easy since almost all of wasmtime
already expected multi-memory to be implemented one day. The only real
substantive change is the `memory.copy` intrinsic changes, which now
accounts for the source/destination memories possibly being different.

cranelift/wasm/src/environ/spec.rs

@@ -387,8 +387,10 @@ pub trait FuncEnvironment: TargetEnvironment {
     fn translate_memory_copy(
         &mut self,
         pos: FuncCursor,
-        index: MemoryIndex,
-        heap: ir::Heap,
+        src_index: MemoryIndex,
+        src_heap: ir::Heap,
+        dst_index: MemoryIndex,
+        dst_heap: ir::Heap,
         dst: ir::Value,
         src: ir::Value,
         len: ir::Value,