implement Display for u5

[tcharding]

No reason not to enable printing u5s.

Fix: #80

[apoelstra]

It looks like these are being displayed as decimal numbers. This is unexpected. They should be displayed as bech32 characters.

[tcharding]

bech32 knows about u5 but u5 does not know about bech32; isn't u5 a stand alone type and hence its Display impl should behave like other standard integer types?

[tcharding]

Review question unresolved but I rebased while thinking about it.

[apoelstra]

I think of u5 as a field element in GF32, definitely not as a standalone integer type. We could rename it if this is confusing.

Do we have Add and Mul implemented on this type, and do they respect field operations or "mod 32" operatons?

To be clearer: this crate has no use for integers mod 32, and I can't think of any crate that does.

[tcharding]

Closing this, see #94