…formation

Field `dbVersion: DbVersion` was removed due to it being inferable from
a `Dialect` instance.

This change provides access to more information about the active SQL
dialect. For example, we get access to Hibernate's knowledge of SQL type
names, which is required to solve the problem described in this issue.

…ad of `DbVersion`

…ype casts

* Increase immutability
* Be concise
* Add documentation and comments

Needed additional code to overcome limitations of Hibernate

Null is a bad idea

…inMetadata

… literal null

Programming with nulls is undesirable.

This is to avoid 2 placeholder types from different contexts
being accidentally considered equal.

- Add documentation
- Implement toString() methods to facilitate reasoning
- Reduce visibility of PropType constructor (static method should be used)

…ions

This is achieved by casting the NULL to the type of the other operand

In general, this approach of asserting that an exception is thrown is
far from the best because it ignores potential exceptions that may be
thrown for unexpected reasons. Being more specific and expecting only a
certain type of exceptions doesn't work either since the exception gets
rethrown at some higher level, effectively becoming a cause of another
exception. Should we be making assertions about the stack trace at this
point? I don't think so.

The converstion itself was also slightly modified:
* Use an unbounded VARCHAR by default instead of setting the limit to
  255 chars, which has no clear basis.
* Optimise for Integers by limiting the VARCHAR length to 10 chars.

…tility

And for all DB back-ends, not just PostgreSQL and SQL Server.

… with string type

Wheels of EQL3 are turning so the workaround is no longer needed.
Moreover, we can now be confident that the null type is never taken for
the string type by mistake, which is vastly important for PostgreSQL.

…iling

* Use a sealed interface with inner classes to facilitate pattern
  matching with switch.
* Instead of inserting type casts conditionally, depending on the
  database, always insert them. The conditional approach likely relied
  on implicit type conversion performed by some DBs. Although,
  endAsInt() worked correctly only with H2, which had gone unnoticed as
  its usage has been scarce.

This assertion reports list contents in case of failure.