Moved all curnel code into curnel/, and split the two kernel modules
into separate files. Now the trusted core and the module/#lang stuff are
in separate files. The #lang is now reprovided by "cur.rkt", which
should also provide core agnostic sugar.
* Various style fixes
* Standardize the proof state interface/naming a little.
* Reorganized base.rkt; now easier to read
* Fixed some mismatch code/documentation
* Renamed "basic" to "standard"; basic was too close to base, and not
really helpful.
* Added the Sartactics library, provides a sassy wrapper around the basic
tactics.
* Added prefix-in, although it probably only works for macros
* Better syntax for define-tactic
