unsafe fn u8to64_le(buf: &[u8], start: usize, len: usize) -> u64
Loads a u64 using up to 7 bytes of a byte slice. It looks clumsy but the
copy_nonoverlapping calls that occur (via
load_int_le!) all have fixed
sizes and avoid calling
memcpy, which is good for speed.
Unsafe because: unchecked indexing at start..start+len