Caqti 2.0.0

[paurkedal]

The addition of MirageOS (#38) will require a new major release. The current status and plans:

• Deprecated definitions will be removed (eb68095b99f4ac820d5815821c48d82816718eff).
• The parts of the API used by drivers and concurrency implementations is re-designed to dispatch between a depedency on unix or using a pure OCaml network stack.
• As a minor braking change, the Caqti_error.retrieve is extended to support the fused request-and-retrieval API of PGX.
• It will include the PGX driver (done, multiple commits).
• It will include MirageOS support (done except for TLS, multiple commits).
• Should modules from the core library be wrapped in a Caqti module and Caqti_ prefixes removed? I tend towards at least postponing this till a later version, though.

[paurkedal]

Now released, but TLS for PGX postponed for an upcoming release. Namespace changes on hold for the moment.