Throwaway prototype in PL/pgSQL

Implements md5 hash-based partitioning across a fixed number of tables
using `postgres_fdw`. Supports `SELECT` and `INSERT` but not (yet) any
`UPDATE` or `DELETE` commands. Also only works if replication is off.
Fix copy-paste typo
Add tests and fix rough edges found during testing

Wanted to see what it was like to write regression tests for an ex-
tension, so getting some practice with my prototype.
Simplify check for view/trigger

Clarify some testing.
Add test for relocatability

Topsie is not (currently) relocatable.
Merge branch 'feature/initial_impl' into develop