Only install mimalloc for bin
Currently building egglog is failing for the web demo because of an
issues installing the mimalloc allocator (https://github.com/egraphs-good/egglog-demo/actions/runs/17182776958/job/48747498611)
I also had issues building mimalloc in CI when building one target of the Python bindings.
This PR changes mimalloc to only be a bin dependency, so its not installed when just
installing the package.
Log proper function type instead of calling them all "function"
Closes https://github.com/egraphs-good/egglog/issues/656 by logging
properly what kind of function is declared, whether its a relation,
constructor, or function.