Add support for PostgreSQL ENUMs (and portable named types)

[emk]

This is a fairly major change, and it breaks the output format for
dbcrossbar-schema (so this is a semver bump). Highlists of this
change include:

• Parsing and printing CREATE TYPE name AS ENUM(...) statements.
• Creating ENUMs in the database if they don't exist. Wrapping the existing dbcrossbar-schema output format in { "named_types": [], "tables": [old_table_format] }.

Internally, this requires:

• Passing portable Schema around instead of Table in most places,
  so that we have access to named_types. - Adding DataType::Named
  to refer to a named type. - Adding DataType::OneOf to refer to a
  string enumeration. - Lots of changes to the postgres-sql and
  postgres drivers.

We designed DataType::Named and DataType::OneOf the way we did in
order to later support the following planned features:

• dbcrossbar-ts should be able to convert type and interface
  declarations into named types. - We want to support PostgreSQL
  native structure types, as an alternative to encoding structures as
  jsonb options.

Note that one_of and named support is fairly limited. Some drivers
will always recursively resolve all named types (like BigQuery), but
PostgreSQL only accepts one_of and named if they correspond to a
CREATE TYPE name AS ENUM(...) statement. These restrictions will be
relaxed over time as we figure out how to handle these cases
correctly, but we want to start by allowing only those cases we know
how to handle cleanly.

Other interesting PostgreSQL changes include:

• TableName is now PgName, because it's used by types and tables.

Many thanks to Tom Caruso for pairing on a tricky set of tests and
test fixes, and to Forrest Wallace and Seamus Abshere for providing
feedback on the design.