reset attributes after taut/2 to reclaim nodes from local hash tables
shorten cases in bdd_restriction_//4
comment on declarations for library(sandbox) and future perspectives
introduce bdd_nodes/2 and make it the building block for other relations

bdd_ites/2 uses too much memory to be useful in larger tasks, since it
essentially duplicates the entire BDD even though only a small subset
of its elements are often needed.
save some space in all tables by not wrapping node IDs in node(...)

Note that 0 and 1 get dedicated IDs (false and true) and therefore
still do not interfere with other IDs, which are always integers.