<:Q> returns -1 for non-existent registers now

 * added a new OPTIONAL behaviour for QRegSpecMachines
 * allows you to implement commands that have an optional Q-Register
   argument that should not be initialized if undefined.
 * Using QRegSpecMachine::fail() you may still check for existence of
   the register conditionally to emulate the QREG_REQUIRED behaviour.
 * Using :Q for checking for register existence makes sense, because
   usually you will want to check for both existence and non-emptyness
   as in :Qq">. So in this common case, you no longer have to
   keep in mind that the register may also be undefined.
 * This finally allows us to create arrays in the Q-Register
   tables without keeping a separate entry for the number of elements.
   E.g. an array.0 to array.N can be iterated like this:
   0Ui <:Q[array.^E\i]:; ! work with element i ! %i>

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