Integrate code from the 2.1a ("fast") branch

[logological]

GPP 2.1a is an incompletely documented and dead branch of the GPP development tree. Most of the differences from the previous version, 2.1, are internal changes to speed up performance when processing particularly complicated macros.

The branch should be imported into the Git repository as a Git branch, and the worthwhile changes should be merged into the master branch.

(This issue was migrated from the old TODO file of 2004-09-19.)

[logological]

The original 2.1a code is now available in the 2.1a branch.