Highlight the UInt128 and UInt256 data types

[rfraposa]

This is two years old in a different repo - moving to here:

From @alexey-milovidov : highlight UInt128, Uint256

Somebody tweeted about it: https://twitter.com/MixBytes/status/1448028864416993282?s=20 mentioning how it's convenient for block chain use cases. It would be good to have a simple user guide on how to use them