Think about enum data types

[krlmlr]

• https://dev.mysql.com/doc/refman/5.7/en/enum.html#enum-using
• https://www.postgresql.org/docs/9.6/static/datatype-enum.html

CC @nbenn.

[krlmlr]

Closing in favor of https://github.com/r-dbi/DBI/issues/203.