Implementation of miniKanren relational (logic) EDSL: PPX extensions

PPX rewriters for writing relational programs more conveniently.

1) ppx_fresh -- a clone of original miniKanren syntax

fresh (x...) goal

expands into

Fresh.numeral (fun x ... -> goal)

2) ppx_distrib for easier generation of type-safe smart constructors and reifiers...

[%%distrib type nonrec _ t = fully_abstract_type type ground = ... ]

3) ppx_repr for testing. Expands REPR(expr) to (string_of_expr expr, expr)