message: rename NewMsg to New

Should have done this when relocating this from container. Now is a good time to rename it before v0.3.x.

Signed-off-by: Ophestra <cat@gensokyo.uk>

container/executable_test.go

@@ -11,7 +11,7 @@ import (
 func TestExecutable(t *testing.T) {
 	t.Parallel()
 	for i := 0; i < 16; i++ {
-		if got := container.MustExecutable(message.NewMsg(nil)); got != os.Args[0] {
+		if got := container.MustExecutable(message.New(nil)); got != os.Args[0] {
 			t.Errorf("MustExecutable: %q, want %q", got, os.Args[0])
 		}
 	}