Split redex-lib into redex-lib and redex-gui-lib.

There are two questions I still have about this pull request:

1. What should I do about the generation of math identifiers.  Right now, it uses mrlib/tex-table, which is in the `gui-lib` package. I could either split that out into a new package, change redex not to use these names, or dynamically load the relevant module.
@rfindler, what do you say?

2. Is this the right place to split?  Everything that uses picts is in `redex-gui-lib`.  Perhaps the line should be "things that use `gui-lib`".  Or somewhere else.  Or there should be a third package?