Consider storing enumerated attributes as atoms

[jdm]

@nox's position is that the current representation wastes spaces and performs worse than storing an Atom. I haven't put any thought into the matter.

[nox]

This was prompted by #9100. Note that all the related button types in that case are short, and would be all inline even if not in the atom global static table. They should anyway because we want to use them as patterns in attribute_mutated.

[nox]

As stated in #9132, <input> already use atoms for its type attribute value.

[nox]

We now do this pretty consistently, no?