[api] Move `Constrexpr` to the `interp` module.

Continuing the interface cleanup we place `Constrexpr` in the
internalization module, which is the one that eliminates it.

This slims down `pretyping` considerably, including removing the
`Univdecls` module which existed only due to bad dependency ordering
in the first place. Thanks to @ Skyskimmer we also remove a duplicate
`univ_decl` definition among `Misctypes` and `UState`.

This is mostly a proof of concept yet as it depends on quite a few
patches of the tree. For sure some tweaks will be necessary, but it
should be good for review now.

IMO the tree is now in a state where we can could easy eliminate more
than 10 modules without any impact, IMHO this is a net saving API-wise
and would help people to understand the structure of the code better.

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