Concise Ast_traverse.sexp_of

[ceastlund]

As tests in the first commit show, Ast_traverse.sexp_of is ridiculously verbose. The output is overwhelmed with empty fields, the fields of location records, and so forth. This feature applies several shorthands to the sexp that make it much more manageable, while still uniquely expressing a value.

1. Longidents are rendered as a single string, unless parsing fails to round-trip.
2. Location.none is elided as ().
3. Other locations are rendered as strings, with suffix <ghost> if loc_ghost=true.
4. Records drop fields containing only the sexp ().
5. Records with only one remaining field named txt or *_desc reduce to that fields' value.

After these changes, the test output goes from so long OCaml won't print it to only a few times larger than the Pprintast output.

[NathanReb]

Looks good to me!