Log Message:
Updated devel/ocaml-extlib to version 1.7.8.

Changes include:
* sync with OCaml 4.12
* breaking change: ExtList.find_map type updated to match stdlib (following \ 
deprecation in previous release)
* breaking change: minimal (recommended) build of extlib now excludes Base64 module
* String: fold_left and fold_right without allocations