Notes on the parser solution set

The parser solution set is actually a solution set for the interpreter, with
all the 'interpret' code removed. If you wish to use this solution set in
building your interpreter, you get (for free!) the necessary changes to the
symbol table. You also get exceptions declared as thrown in exactly those
functions which (in the interpreter solution set) actually threw them. In other
words, the "header" of an interpret() method tells you exactly what exceptions
are thrown in the body of that method, even though we've removed the body of
the method.

The solution code uses BigIntegers to handle arbitrary-length integers, a
feature which is optional.