Call-by-value

[rolyp]

Drop "demand-indexed" evaluation; too much of a distraction and not directly relevant to what we want to do. Closely related to backwards slicing, but not equivalent in that it doesn't automatically compute slices of functions.

We will retain tries-with-variable-binding as a nice approach to pattern matching, but will no longer need "top" tries, which are a not-entirely-straightforward special case at the moment (see #74).

• delete Top tries and argument tries
• new eval_; drop EnvEntry, evalArgs
• EvalKey, Result and Results no longer required
• letrec can only define recursive functions, not values; revisit closeDefs
• lookup which returns an environment
• inline PrimOp.invoke and delete demands of primitive ops
• do I still need merge/LVar semantics? -- no