Tag functions with attributes pure, const, or noreturn.

* src/bdd.h (__purefn, __constfn, __noreturnfn): Define
new macros.
* src/bdd.h, src/bddio.c, src/bvec.h, src/imatrix.h: Use them
to tag many functions as suggested by -Wsuggest-attribute=pure,
-Wsuggest-attribute=const, -Wsuggest-attribute=noreturn.