Add Persistable instances for HDBC.
Divide abstract record type constraints definitions into DB-record module.

--HG--
rename : schema-th/src/Database/Record/FromSql.hs => DB-record/src/Database/Record/FromSql.hs
rename : schema-th/src/Database/Record/KeyConstraint.hs => DB-record/src/Database/Record/KeyConstraint.hs
rename : schema-th/src/Database/Record/Persistable.hs => DB-record/src/Database/Record/Persistable.hs
rename : schema-th/src/Database/Record/ToSql.hs => DB-record/src/Database/Record/ToSql.hs