Make expressions persistent across edits

[dimitar-asenov]

At the moment, when an expression is edited, its nodes are recreated after every parse. In most cases the new expressions is only minimally different than the old one, and it is desirable to keep the existing nodes, instead of creating new ones. This will help with associating data with expression nodes (i.e. comments).

One approach might be to create the tree corresponding to the new expression and then scan for similarities and only merge the differences.