Rename {find,canonicalize} to unsafe{Find,Canonicalize}

Hello, and thanks for the good work.

This is a design proposal

Both `find` and `canonicalize`, as defined, are abstraction breaking, because the data structure is not in a sound state if you don't call `rebuild` previously. My proposal, in keeping with the spirit of Haskell of correctness first is that:

- the functions named `find` and `canonicalize` should call `rebuild` before querying the data structure (note: in the case where the data structure is already rebuilt, then the work list is empty, therefore `rebuild` will be very fast anyway)
- for the sake of performance tuning, direct queries to the data structure, called `unsafeFind` and `unsafeCanonicalize` would be introduced. With the documentation explaining that the onus is on the programmer to only call them on a “rebuilt” E-graph.