more efficient defs

[c-cube]

definition of a constant should be:

type def =
  | Rule of rule
  | Fun of builtin_fun

type defs =
  | Def_cst of expr (* the constnat itself is defined *)
  | Def_n of def list

type cst = {
  cst_defs: defs;
  ...
}

This way, a = … is efficient because it erases previous bindings, etc. instead of accumulating.

Also, pat := foo should erase any previous rule of Head[pat] that is matched by pat (if the LHS is matched, it means the rule is subsumed and will never fire).