add mem.Count function

The [`bits.CountByte`](https://github.com/segmentio/parquet-go/blob/main/internal/bits/count_amd64.s) 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:

```go
package mem

func Count(b []byte, v byte) int
```