Refactor alphabet checking in Bio.Seq methods into a decorator function.

This is just a refactor of the alphabet check for several `Seq` methods. This patch puts all alphabet function into a single decorator function.

I thought this would make it easier for us to add more alphabet-specific methods later on.

As a side note, I was actually thinking of merging `Bio.SeqUtils` with `Bio.Seq` (i.e. making the functions from `SeqUtils` as methods of `Seq`). There's more to discuss about this, of course, but in short, I feel like it makes our `Seq` objects more feature-complete and easier to use (no need to import `SeqUtils` if you want to check GC percentage, for example).