add mem.Count function

[achille-roussel]

The bits.CountByte function in https://github.com/segmentio/parquet-go is a good candidate to bring to https://github.com/segmentio/asm

Here is a suggestion for the signature:

package mem

func Count(b []byte, v byte) int