Eliminate boilerplate for slick-pg's enumeration support

[luksow]

Enum support in slick-pg needs some boilerplate registration. It'd be great to avoid it:

https://github.com/tminglei/slick-pg/blob/master/src/test/scala/com/github/tminglei/slickpg/PgEnumSupportSuite.scala

Nice to have - both for Scala's Enum and enumeratum.