ocamldoc: Revert previous commits following review from gasche (see [1])

[1]: http://caml.inria.fr/mantis/view.php?id=6274#c10766

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Revert "ocamldoc: (bugfix) don't duplicate printing of object types fields"
This reverts commit 501f15c017c573b0978f8771e78e4157623f7090.

Revert "ocamldoc: better support (printing) of object types in every backend."
This reverts commit 68fc982c9985636ed212c22699fa4a6bea8be7f0.
ocamldoc: information can now come from type_manifest
ocamldoc: (minor) rewrite [Odoc_str.string_of_type]
ocamldoc: factorize redundancy between objects and records fields handling