Postgres UUID can't be used in values_ (+ fix)

[roberth]

The postgresql backend generates UUID strings without a type. Most of the time PostgreSQL is clever enough to infer the type, but when used in values_, it only infers locally, leading to missing cast errors down the line. This can be fixed by always appending ::uuid to UUIDs.
I haven't made a PR because I don't know where/how this should be changed. beam-postgres seems to borrow a lot of value syntax from postgresql-simple, which I'm not familiar with.
For my own application, I was able to change the syntax in my UUID newtype.

instance HasSqlValueSyntax PgValueSyntax (Id a) where
  sqlValueSyntax (Id uuid) = PgValueSyntax $ fromPgValue (sqlValueSyntax uuid) <> emit "::uuid"

[kmicklas]

This looks like a postgres-simple bug but we could work around it by replacing DEFAULT_SQL_SYNTAX(UUID) with your instance.

[kmicklas]

I've reported lpsmith/postgresql-simple#277 to see if it's possible to get this fixed upstream.