equal: Generate presence checks for bytes fields, where necessary

[misberner]

Updates the equal code generation to correctly generate presence checks for bytes fields where necessary, by distinguishing []byte(nil) from []byte{}. See #52 for a more detailed description.

Fixes #52 .

[vmg]

Thanks for your attention to detail! I don't think we would catch any of these corner cases organically since we don't use Equals in Vitess ourselves. ❤️