Deletes namespace API (#1018)

Formerly, we introduced `wazero.Namespace` to help avoid module name or import conflicts while still sharing the runtime's compilation cache. Now that we've introduced `CompilationCache` `wazero.Namespace` is no longer necessary. By removing it, we reduce the conceptual load on end users as well internal complexity. Since most users don't use namespace, the change isn't very impactful.

Users who are only trying to avoid module name conflict can generate a name like below instead of using multiple runtimes:

```go
moduleName := fmt.Sprintf("%d", atomic.AddUint64(&m.instanceCounter, 1))
module, err := runtime.InstantiateModule(ctx, compiled, config.WithName(moduleName))
```

For `HostModuleBuilder` users, we no longer take `Namespace` as the last parameter of `Instantiate` method: 

```diff
 	// log to the console.
 	_, err := r.NewHostModuleBuilder("env").
 		NewFunctionBuilder().WithFunc(logString).Export("log").
-		Instantiate(ctx, r)
+		Instantiate(ctx)
 	if err != nil {
 		log.Panicln(err)
 	}
```


The following is an example diff a use of namespace can use to keep compilation cache while also ensuring their modules don't conflict:

```diff

 func useMultipleRuntimes(ctx context.Context, cache) {
-	r := wazero.NewRuntime(ctx)
+	cache := wazero.NewCompilationCache()
 
 	for i := 0; i < N; i++ {
-		// Create a new namespace to instantiate modules into.
-		ns := r.NewNamespace(ctx) // Note: this is closed when the Runtime is
+		r := wazero.NewRuntimeWithConfig(ctx, wazero.NewRuntimeConfig().WithCompilationCache(cache))
 
 		// Instantiate a new "env" module which exports a stateful function.
 		_, err := r.NewHostModuleBuilder("env").
```

Signed-off-by: Takeshi Yoneda <takeshi@tetrate.io>

builder_test.go

@@ -327,11 +327,11 @@ func TestNewHostModuleBuilder_Compile_Errors(t *testing.T) {
 // TestNewHostModuleBuilder_Instantiate ensures Runtime.InstantiateModule is called on success.
 func TestNewHostModuleBuilder_Instantiate(t *testing.T) {
 	r := NewRuntime(testCtx)
-	m, err := r.NewHostModuleBuilder("env").Instantiate(testCtx, r)
+	m, err := r.NewHostModuleBuilder("env").Instantiate(testCtx)
 	require.NoError(t, err)
 
 	// If this was instantiated, it would be added to the store under the same name
-	require.Equal(t, r.(*runtime).ns.Module("env"), m)
+	require.Equal(t, r.Module("env"), m)
 
 	// Closing the module should remove the compiler cache
 	require.NoError(t, m.Close(testCtx))
@@ -341,10 +341,10 @@ func TestNewHostModuleBuilder_Instantiate(t *testing.T) {
 // TestNewHostModuleBuilder_Instantiate_Errors ensures errors propagate from Runtime.InstantiateModule
 func TestNewHostModuleBuilder_Instantiate_Errors(t *testing.T) {
 	r := NewRuntime(testCtx)
-	_, err := r.NewHostModuleBuilder("env").Instantiate(testCtx, r)
+	_, err := r.NewHostModuleBuilder("env").Instantiate(testCtx)
 	require.NoError(t, err)
 
-	_, err = r.NewHostModuleBuilder("env").Instantiate(testCtx, r)
+	_, err = r.NewHostModuleBuilder("env").Instantiate(testCtx)
 	require.EqualError(t, err, "module[env] has already been instantiated")
 }