Wipe out the old (bison/flex generated) parser.

Wipe out the old (bison/flex generated) parser.

Bison generated parser has a number of problems:

1. Bad performance. Parsing a sum seems to be O(N^{2 + a}) (a > 0).
   For example, parsing a sum (actually, a univariate polynomial) of 32768
   terms takes about 90 sec. on my box, parsing a sum of 10^6 terms takes
   "eternity".
2. The user is expected to provide list of all symbols in the expression.
   Often this is very annoying (and useless), sometimes it is not possible
   at all.
3. Parser is not reentrant (bison *can* produce reentrant parsers, but that
   won't solve other problems).
4. Parser is difficult to extend.

Since the new parser handles almost everything (useful) as the old one, let's
remove the latter.