Introduce "custom types"

[oyvindberg]

• to handle all the weird edge cases around (arrays of) postgres types.
• enables cross-compilation to scala.js where you generate one version of the code with just json codecs, and one version for backend with db code
• now have somewhere to put orphan instances which have gone into parent package object for now. note that there are still some left still

for later:

• unary custom types should be encoded as AnyVals
• all other usage of PGobject should also receive custom types (supporting all types will be on-going)