nLeadingBitsToCount

assert(nLeadingBitsToCount(0xF0u, 3u) == 32 - 5);
assert(nLeadingBitsToCount(0x700u, 3u) == 32 - 8);

foreach(uint i; 1 .. 32)
    assert(nLeadingBitsToCount(uint.max, i) == i);