Support non-contiguous edits (for deletions)

[ethan-leba]

Currently the tree-edit syntax generation and rendering relies on the
assumption that all modifications of the syntax tree would operate on a
contiguous region (adjacent siblings).

For example, if we have ("(" identifier "," comment [identifier]) and perform
a deletion, we'd like the result to be ("(" identifier comment), and the
comment should be left unmodified.

Related to #18