Add a "rename" operation to signatures

[jonsterling]

This operation should take every free symbol (basically, the operator ids) and rename it by some operation (such as a qualifying prefix like fn x => "mylib#" ^ x, etc.). This shall properly be applied in each declaration, etc.

We can use this as a poor-man's support for namespacing. Longterm, we can introduce a new implementation of the SYMBOL signature that includes some path structure in it.

We might want to add an operation to ABT to support mapping a function over the free names in a term, since currently you have to manually substitute each one, which could be fairly inefficient.