Note that we only use this for change detection.
The previous implementation invoked `MD5FromReaderFast` that created a MD5 has from 8 64 bytes chunks in the file, which is obviously very fast. The new implementation creates the hash from the entire file and ... seems to be even more effective:
```
name old time/op new time/op delta
HashImage-10 9.45µs ±21% 10.89µs ± 1% ~ (p=0.343 n=4+4)
name old alloc/op new alloc/op delta
HashImage-10 144B ± 0% 8B ± 0% -94.44% (p=0.029 n=4+4)
name old allocs/op new allocs/op delta
HashImage-10 4.00 ± 0% 1.00 ± 0% -75.00% (p=0.029 n=4+4)
```
