Add Expr.mathParse method.
Rewrites with facts now appear as "use" rather than "fact" in the rule menu.
Convert existing code and proofs to use rewriters with facts.  Parse
textual facts with mathParse for convenience.
Fix bug in demo code.
Add Toy.hypIsLess.
Add debugger DIV to proofbuilder.html.
Reset the step editor in case there is an error in tryRule.
This resets the rule menu, selecting nothing in particular.
Add rules.addAssumption and various simple facts.
Add debugging support to Toy.getResult, the fact prover.