Convert ParameterType to enum

[morozov]

Q	A
Type	improvement

Positive outcomes:

1. The driver-level statement API is now more robust. The UnknownParameterType exception is no longer possible since all possible values of the parameter type are constrained by the enum.
2. The driver-level parameter types and the wrapper-level array types which were previously described by a single int type are now distinct (ParameterType and int respectively). This is important since the wrapper-level array types have non-obviously limited support. For instance, they can be used in Connection::executeQuery() but not in Statement::bindValue().

Negative outcomes:

1. The PHP types describing parameter types became more complex (which reflects how complex they really are).

As the next step, we can simplify the type of parameter types by making them non-nullable. There is already a default of "string" (see #5550).

TODO:

• Mark Connection::ARRAY_PARAM_OFFSET internal #5549.

[morozov]

This chunk is dedicated to all fans of union types.