[serlib] Large refactoring, ppx_hash/compare support, better yojson

[ejgallego]

We perform a very large refactoring of serlib, moving all the Obj.magic hacks to a more principled solution based on interfaces, mainly exported by SerType. Moreover, we use a similar technique to improve the handling of generic arguments.

Using this new infrastructure, we provide support for ppx_hash / ppx_compare support for most data objects. We have chosen to ignore locations by default, see the ignore attributes to tweak this.