Change type of Prepare_failed exception.
Extend describe to one-shot queries.
Replace Tuple.raw with Tuple.other, add Param.other.
Handle Oid exception in PostgreSQL backend.
Rename Caqti_types to Caqti_describe.
Add Caqti_describe.string_of_typedesc.
caqtiq: Use describe to format output, make it CSV, etc.
Remove Tuple.length; subsumed by describe.