[wish] loc API

[gares]

the loc datatype is opaque, it would be nice to have

• a predicate to get file, line, chars... out of a loc (HB prints the loc to a string and then chops it with regexp)
• a gadget type located loc -> A -> A
  • have the parser optionally insert it
    • use it in the type checker to report errors more precisely
  • utility predicate to copy any term and remove it

at the API level, smart constructors could take an optional ?loc and put the located wrapper

to be understood if there are other uses of this located, maybe it could become info B -> A -> A and then (info (loc L) T) or (info [loc L, typ Ty] T) the way it is used to attach a loc and a type to T, and eventually info list prop -> A ->A to let one write

p (info Info T) :- Info => p-rec T.