We do not actually every parse negative values in there, as a negative value is a combination of a 'Negate' and 'UInt' expression.
However, for patterns and constant, it'll be simpler to parse whole Int values as there's no ambiguity with arithmetic operations
there. To avoid confusion of having some 'Int' constructors containing only non-negative values, and some being on the whole range,
I've renamed the constructor to 'UInt' to make this more obvious.
