Optimized Armv8 methods

[jakobnissen]

The new-ish Apple computers don't have x86 instructions. Make sure to write a fast implementation for those.
Edit: A few notes:

• Instead of the vpshufb instruction, use tbl. For vec_generic, tbl can take two LUT directly in one instruction, making the code significantly simpler than the x86 one, where the entire function could be:

(1 << (x >>> 5)) & tbl(x & 0b00011111, lut1, lut2)

• There is no equivalent to pmovmskb. Perhaps the fastest is just to cast to uint128 and do a direct count of leading zeros (for trailing zeros, do rbit + clz - probably LLVM already does this for you, check)