Add a compilation pass

[erikrose]

Add a compilation pass for rulesets. This will bring several improvements:

• Catch errors early: for example, dom() rules that don't assign a type.
• Compute rule prereqs only once.
• Support and() rules with more complex operands. (Compiling down to simpler rules is much easier than coding it all into AndLhs.)

I see us inserting a CompiledRuleset class chronologically between Ruleset and BoundRuleset. Callers can compile a ruleset manually if they wish, or they can just call Ruleset.against() as now, and the compile will happen implicitly (and then get thrown away).