Defined the rest of the keys

Completed the key definitions for the calculator.
Changes to allow reporting of overflow

Made the round-to-ultimate-precision function return two values, the
second of which indicates whether or not the number is in bounds.
Various tweaks

Allowed the definition of narrow and compound keys.

Documented the categories of keys, and their return codes.

Changed the default modes under which a keypress is available.

Put in a collision detector to make sure we didn't have multiple keys
defined for the same press.

Added some more support macros for the key definitions.
Minor changes

Added a function to set angle units, and documented the modes of
operation of the calculator itself.
Minor tweaks

Added a condition to report overflows.  Added some flag manipulation
functions, and more support functions related to keypresses.