Adds MemoryDefinition to CompiledModule and Memory (#817)

It is more often the case that projects are enabling a freestanding
target, and that may or may not have an exporting memory depending on
how that's interpreted. This adds the ability to inspect memories
similar to how you can already inspect compiled code prior to
instantiation. For example, you can enforce an ABI constraint that
"memory" must be exported even if WASI is not in use.

Signed-off-by: Adrian Cole <adrian@tetrate.io>

internal/wasm/module_test.go

@@ -787,7 +787,7 @@ func TestModule_buildFunctions(t *testing.T) {
 	instance := &ModuleInstance{Name: "counter", TypeIDs: []FunctionTypeID{0}}
 	instance.BuildFunctions(m, nil)
 	for i, f := range instance.Functions {
-		require.Equal(t, i, f.FunctionDefinition.Index())
+		require.Equal(t, i, f.Definition.Index())
 		require.Equal(t, nopCode.Body, f.Body)
 	}
 }