(PEG-109) - HorseWhisperer fails to parse negative arity

HorseWhisperer fails to process action arguments when the arity is
negative. Such values specifies the minimum number of arguments of a
given action.

This commit fixes HorseWhisperer; in case of an action with negative
arity, the parse function will now correctly validate the number of
arguments.
We use that variable in many different blocks. A more meaningful name helps reading.