These are an order of magnitude quicker than the previous
implementations:

A relative comparison of each, measuring scanning a 1G file.

    Reading 1G (1.0000000009313226GiB)

             std.mem.sliceTo: 281.232ms
          vectorized.sliceTo: 24.769ms
                      strlen: 24.291ms

           std.indexOfScalar: 229.016ms
    vectorized.indexOfScalar: 24.685ms
                      memchr: 24.958ms