Hash file size in the first pass

[jRimbault]

Hi, just wanted to let you know of a possible small optimization, but since it's in the early phases it could have a large impact.

During the first partial hash, send the file size to the hasher as well (similarly to how slices are hashed in the standard library):

https://github.com/jRimbault/yadf/blob/8fe56366b6f4c9ffc821a48dfd3afeb56648131e/src/fs/hash.rs#L24-L27

I don't know if it would apply as well to your algorithm.