Add `count_leading_zeros` function

Add the following function in `src/util.hpp`:

	template<typename N> unsigned int count_leading_zeros(N n);

This function returns the quantity of leading zero bits in `n` (i.e.,
the quantity of bits in `n`, minus the 1-based bit index of the most
significant 1 bit in `n`).
Add `count_leading_zeros` unit tests

Add unit tests for the `count_leading_zeros` function that was added to
`src/unit.hpp` in this commit’s parent commit.