New explicit argument &key nxt to all protocol functions
New explicit argument &key nxt to all protocol functions

Allows users to specify which NXT to connect to.  The default value for
this argument is still the implicit *nxt* setting, but users working with
multiple devices or threads can now avoid use of the special variable.

Example:
  (nxt:nxt-play-tone :nxt <some-other-nxt> :frequency #x500 :duration #x100)

woudshoo/cl-nxt