Fix a bug in sto-

The sto- had the arguments reversed on the subtraction.  Changed two
other commutative functions for consistency of presentation.
Added compound keys

Added the ability to handle compound keys to the interpreter entry
point.
Added a function to return only compound keys

Needed the ability to retrieve only compound keys (keys such as STO +
that combine two operation keys to make a new behaviour).
Move everything to a package

It's time to put all the functions we've defined so far into a
package, 'hp67-internals'.  Those packages that own symbols that might
be useful to the UI export them directly with the (export) function,
rather than doing it in the defpackage.