Set compilation option and calculator startup default

For development purposes, we'd like the compiler to generate debug
code.

The HP-67 starts up in degrees mode, so I've changed the mode
structure startup.
Changes in support of token input

There have been some changes to how tokens (things that together
assemble a single directive, such as the number keys) are processed.
The engine code is modified to conform to these changes.
New modelist defaults

I've now made all keys active in all modes by default, and the
modelist allows the programmer to specialize a key.
Bugfixes and compilation options

Set the compiler to generate debug code.

Fixed bugs in the behaviour of some string-parsing code.  One was
failing to set its second return value, and another didn't correctly
handle the case of a trailing dot on a number.
Moved a few constants into their own file

Put some constants here, as they are used in more than one place.
Added the new constants.lisp

Put the new constants.lisp module into the project, with the
appropriate dependencies.
Added compilation option, changed token behaviour

Set the compiler to generate debug code.

Allowed keys to be tagged as being token keys so that they could be
handled specially.  Allowed some modelists to go to their defaults.
Added a structure to handle token assembly

As number tokens arrive, they have to be put together to make a single
input value.  The CHS, EEX, and dot keys must be correctly handled.
This change adds a structure to do this work.