Introduce blocking variants.
Fix bug in box_open_afternm + Improve readability of C code.

If we write

	EXP != 0

and EXP is a very large term, it is hard to see what it gets checked against, so we rewrite it as

	0 != EXP

so we instanly see what we check against. While here, teset against 0 != crypto_box_afternm(…)
Introduce timing for precomputed values.
Update changelog.