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>

config_test.go

@@ -853,12 +853,12 @@ func TestCompiledCode_Close(t *testing.T) {
 	for _, ctx := range []context.Context{nil, testCtx} { // Ensure it doesn't crash on nil!
 		e := &mockEngine{name: "1", cachedModules: map[*wasm.Module]struct{}{}}
 
-		var cs []*CompiledCode
+		var cs []*compiledCode
 		for i := 0; i < 10; i++ {
 			m := &wasm.Module{}
 			err := e.CompileModule(ctx, m)
 			require.NoError(t, err)
-			cs = append(cs, &CompiledCode{module: m, compiledEngine: e})
+			cs = append(cs, &compiledCode{module: m, compiledEngine: e})
 		}
 
 		// Before Close.