Use lazy-require in the simplify code

[pavpanchekha]

This PR removes the dynamic-require calls in simplify.rkt and replaces them with lazy-require. I've tested and this seems to work with both regraph and egg-herbie, and if egg-herbie is missing it only loads and runs regraph.

Another benefit of this is it declares the dependency somehow to Racket, whereas dynamic-require doesn't. I'm not sure what this actually does for us but it's probably good.

[oflatt]

This is good, I tested it out and it works great! It makes it much cleaner.