Makes wazero.CompiledCode an interface instead of a struct (#519)

This makes wazero.CompiledCode an interface instead of a struct to
prevent it from being used incorrectly. For example, even though the
fields are not exported, someone can mistakenly instantiate this
when it is a struct, and in doing so violate internal assumptions.

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

builder_test.go

@@ -344,8 +344,9 @@ func TestNewModuleBuilder_Build(t *testing.T) {
 
 		t.Run(tc.name, func(t *testing.T) {
 			b := tc.input(NewRuntime()).(*moduleBuilder)
-			m, err := b.Build(testCtx)
+			compiled, err := b.Build(testCtx)
 			require.NoError(t, err)
+			m := compiled.(*compiledCode)
 
 			requireHostModuleEquals(t, tc.expected, m.module)