`uint` type

[giann]

https://github.com/buzz-language/buzz/issues/306 is planned to extend int size but we could also add an unsigned integer type:

• Extends integer size for most usage (empirically we find that you hit the integer size limit in the positive rather than the negative)
• Enforce the fact that an integer must be positive
• BUT: adds a new edge case that can break at runtime when the value is pushed below 0. But we could simply overflow.
• BUT: adds complexity when we will want to do arithmetic between int and uint