Attempt to tidy-up some proof scripts. Much of this has come from CakeML.

Removed Unicode and tried to eliminate lines longer than 80 characters.

Also added a few new constants to listTheory: INDEX_FIND, FIND and INDEX_OF.

Added "nub" to listSyntax.
Add a FOREACH constant to state_transformerTheory.
Update for compatibility with the latest version of L3.