Unify conversions to [u8]

[tomusdrw]

Currently you can do Uint::to_big_endian(), but for hashes it's hash.0
Would be good to have a single way of doing such conversions.

Something to consider as well:

to_big_endian(&self) -> [u8; LEN]
copy_big_endian(&self, &mut [u8]);

It's kind of annoying to do:

let mut bytes = [0u8; 32];
uint.to_big_endian(&mut bytes);
bytes.to_vec()

each time you want to convert uint into bytes (for instance for custom serialization)