external/core: adding .ml files for the tags
stags: better support for standard library files for ocaml code

Summary: make tags should work better for people other than me :)

Test Plan: make tags

Reviewers: pieter, lisp

Reviewed By: pieter

CC: platform-diffs@lists

Differential Revision: https://phabricator.fb.com/D672861
* h_program-lang/tags_file.ml: threshold_long_line