Ignore OCaml generated files

[xrchz]

Building linksem produces lots of {cmi,cmo,cmx,ml,o} files -- could these be gitignored?

[emersion]

Be careful when gitignore-ing .ml files - some are not auto-generated.

Note that using ocamlbuild makes this a little more bearable as all build files are in a separate directory (apart from .ml files produced by Lem).