Moebius and Mertens functions

[Bodigrim]

This PR aims to fulfil Step 2 from #60: sublinear summation of Mobius function (so called Mertens function).

• Introduce a special type for values of Mobius function. Mobius function returns only in -1, 0 or 1, so it is convenient to denote them by three-valued enum.
• Write a specialised, space-effective and fast version of sieveBlock for Mobius function. It is 5x faster than generic sieveBlockUnboxed.
• Implement sublinear Mertens function with time and space complexity O(n^(2/3)).

Reference: https://arxiv.org/abs/1610.08551v2