Eliminate extraneous allocation and dynamic dispatch in `PlainEditor`

[mwcampbell]

I started by wanting to eliminate Arc<str>. Then I noticed that vello_editor was allocating temporary Vecs for the PlainEditorOps. Finally, it occurred to me that we could probably do something altogether more efficient than PlainEditorOp. I think my refactored version, with separate functions in PlainEditorTxn, will be nicer to maintain as well.

[xorgy]

Thanks, this looks great. I'll give it a quick check. This is basically exactly what I planned to do (the enum thing was a mistake anyway).