It is not a library; it does not contain code, but instead is a core
part of heap_lang.

This reverts commit 3cc38ff6.

The reverted pure hypotheses and variables appear in the wrong order.

Since strip_later is doing a good job stripping laters in the conclusion,
these tactics are thus no longer needed. Also, wp_finish now properly
converts the result in a primitive viewshift in case it is not a weakestpre.

Also remove some superfluous map_ prefixes.