Add a ppx for snarky with_label

[mrmr1993]

This PR

• adds [%with_label label], that expands to with_label (label ^ ": " ^ __LOC__)
• adds let%snarkydef name arg = ..., that expands to let name arg = with_label ("name: " ^ __LOC__) ...
• replaces the simplest cases (with_label __LOC__ at the top of a function definition) with let%snarkydef ... = ....
  • The diffs are overinflated by the formatter after this change; git diff -w (or here) gives a much more representative view of what is actually different.

Checklist:

• Tests were added for the new behavior
• All tests pass (CI will check this if you didn't)
• Does this close issues? List them:

[imeckler]

This is so cool!