Implement pretty-printing of generated .ml code

Implement pretty-printing of .ml code generated in `piqic-ocaml` instead of
relying on external pretty-printer (camlp4o).

Pretty-printing is now enabled by default which makes `--pp` flag obsolete.

Deprecation warning added for `--pp` and `--keep-tmp-files` flags.
Fix: 'piqic-ocaml --normalize-names' should be true by default