OCL25 — OCL 2.3: Message support hard to consume

The support for messages uniquely requires dedicated concrete syntax: ^, ^^, ?

This makes provision of Essential OCL tooling without messages and Complete OCL tooling with messages hard.

The operators are hard to remember and inconsistent with OCL where "forAll" is favoured over upside-down A.

Suggest replace ^,^,? by OclElement::hasSent(), OclElement::messages() and Message::Unspecified (possibly just null), so that Messages can be modularized as an Standard Library extension of additional types, operations and attributes only. No concrete syntax change.