Function u8to64_le

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