`Uint256` support with validation

[mempirate]

Use case

Support the Uint256 data type with validation enabled for [u8; 32]. We currently have to turn off validation to use this workaround #48, which is not ideal.

Describe the solution you'd like

Seamless Uint256 support for [u8; 32] serialization/deserialization.

[slvrtrn]

It seems that deserializing it into a slice of bytes would allow further post-processing on the app side, so let's add it.

A question: are you using an unreleased version of the client from a specific commit? I'd be happy to hear any other feedback regarding schema validation.

[abonander]

{U}Int256 support is implemented in the following PR: #352

Please feel free to review and try it out!