Make serde optional

[riley-ashton]

I added a cargo feature to make serde optional, but default. Since the default feature set includes serde, it is opt out and should not break any crates that depend on this one.

There was problems building the Armenian algorithm, which I am not using, but I had to correct to get this crate to build. I don't know if the casts from usize to u32 are inappropriate or not, since they could overflow, but I don't think your cursors would require more than 32-bits. Another fork removed the Armenian algorithm, probably because they were having problems too.