Servant Shared

This is a generic test that produces an executable for each supported backend library:

• servant-hasql: PostgreSQL database via the hasql library.
• servant-mysql-haskell: MySQL database via the mysql-haskell library.
• servant-postgres-wire (default): PostgreSQL database via the postgres-wire library.

NOTE: the shared directory here is a full copy of the same directory within the warp framework dir. TODO: wait for TFB to add support for language scope dockerfile before moving shared up into the Haskell language dir so both servant and warp can re-use the same modules.