Support UUIDs in PK stored in binary form

[janedbal]

Previously:

• MySQL was working, but emitted warnings like Invalid utf8mb4 character string (assuming utf8mb4 connection encoding)
• SQLite was silent, but not preloading at all
• Postgres was failing hard (Character not in repertoire: 7 ERROR: invalid byte sequence for encoding "UTF8": 0x8d)

[janedbal]

Fix of broken ORM: doctrine/orm#12042

[janedbal]

Another ORM bug detected by our improved testcases: doctrine/orm#12130