Design editing API

[matklad]

Currently, the editing API is just a whole bunch of functions here: https://github.com/matklad/tom/blob/5a1b98e0b0b13529a917a9c2212f6813a9eb1948/src/edit.rs

I am not sure that this is the best way to arrange this API. I think for creation API we might want to change doc.new_entry_from_text/doc.new_entry to ast::Entry::from_text(&mut doc)/ast::Entry::new(&mut doc). We might want to add builders for complex types like tables.

We certainly need mode AST-based editing APIs: EntryOwner::add_entry(doc, entry, position), EntryOwner::remove_entry ....