This adds PostgreSQL numeric mapping to and from
github.com/shopspring/decimal.
Makes pgtype.NullAssignTo public as external types need this functionality.
Begin extraction of pgtype testing functionality so it can easily be used by
external types.
Because reading a record type requires the decoder to be able to look up oid
to type mapping and types such as hstore have types that are not fixed between
different PostgreSQL servers it was necessary to restructure the pgtype system
so all encoders and decodes take a *ConnInfo that includes oid/name/type
information.
