Preprocessor: 1) Rationalize the "fixed atom" scheme, 2) remove redundant...

Preprocessor: 1) Rationalize the "fixed atom" scheme, 2) remove redundant lookups when the text is already available.

This simplification is a prelude to eliminating what I appear unnecessary
symbol inserts into tables when tokenizing in the preprecessor, which
show up as taking notable time. (Performance issue.)  It also simply makes
the preprocessor easier to understand, which it is badly in need of.