Name a few more types.
Add a new PGSQL_FLAGS macro.

Allows you to specify default flags to apply to all subsequent
PGSQL calls. Maybe needs a better name and syntax.
Unravel custom types (eg, enums).
Add a new PGSQL_ENUM("typname[=enumname][:Constr_prefix]) macro.

This macro creates a new ocaml typname based on the enumname in
the database with matching constructors and string conversion
functions.  Not fully tested. Uses Ast because <:str_item<>> is a
pain.

Does not work in all cases; will fail if the enum contains
a character that is illegal in variant constructors, such as '/'.