Reduces exports and prepares ReleaseModuleInstance for testing (#327)

This removes more exports and adds a toehold test for
`ReleaseModuleInstance` so that it can later be completed for both
wasm and host modules.

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

internal/wasm/host_test.go

@@ -5,6 +5,8 @@ import (
 	"testing"
 
 	"github.com/stretchr/testify/require"
+
+	"github.com/tetratelabs/wazero/wasm"
 )
 
 func TestMemoryInstance_HasLen(t *testing.T) {
@@ -448,3 +450,38 @@ func TestMemoryInstance_WriteFloat64Le(t *testing.T) {
 		})
 	}
 }
+
+func TestStore_NewHostModule(t *testing.T) {
+	s := newStore()
+
+	// Add the host module
+	_, err := s.NewHostModule("test", map[string]interface{}{"fn": func(wasm.ModuleContext) {}})
+	require.NoError(t, err)
+
+	// Ensure it was added to module instances
+	hm := s.moduleInstances["test"]
+	require.NotNil(t, hm)
+
+	// The function was added to the store, prefixed by the owning module name
+	require.Equal(t, 1, len(s.functions))
+	fn := s.functions[0]
+	require.Equal(t, "test.fn", fn.Name)
+
+	// The function was exported in the module
+	require.Equal(t, 1, len(hm.Exports))
+	_, ok := hm.Exports["fn"]
+	require.True(t, ok)
+
+	// Trying to register it again should fail
+	_, err = s.NewHostModule("test", map[string]interface{}{"host_fn": func(wasm.ModuleContext) {}})
+	require.EqualError(t, err, "module test has already been instantiated")
+}
+
+func TestHostModule_String(t *testing.T) {
+	s := newStore()
+
+	// Ensure paths that can create the host module can see the name.
+	hm, err := s.NewHostModule("host", map[string]interface{}{"host_fn": func(wasm.ModuleContext) {}})
+	require.NoError(t, err)
+	require.Equal(t, "HostModule[host]", hm.String())
+}