Better clearing of the iostream pointers.

Previously, subclasses were required to clear dvi_file.
Enlarging Τεχ’s constants.

The changes are made via a tangle change file.

$tangle -underline tex.web tex.ch
Moving buffers into std::vectors.

The size of the buffers are still static. They have simply been moved explicitly onto the heap. Non-main threads often have a reduced stack size. The previous size of the typesetting classes could cause a problem.