Implement graph transformations to check how usable the infrastructure is

I think it would be useful to develop graph transformations, either in examples or in a library inside pytato, as a usability test of the infrastructure. Examples I can think of:

Easy:

• Add/delete a node to a computation (returning a new computation)
• Rename an arbitrary node (including placeholders)

Medium:

• Ad-hoc pattern-matching transformations (such as replacing AA^{-1} with identity)

Hard/advanced:

• A term rewriting system