Fix u64 overflow when decoding

[Racum]

I have a scenario where I need to test an ID not knowing before hand if is a valid Sqid or not, and, in some cases when an ID looks valid, but results in elements bigger than the maximum u64 value, the decode() function panics! For example, with the ID 0J4AEXRN106Z0.

To fix this I made to_number() return an Option<u64> (instead of just u64), and only push the value on decode() in case of Some().

By the way, I’m adding support to Sqids to an upcoming version of my ID parser/debug CLI tool, have a look: https://github.com/racum/uuinfo