Restrict comparisons (< and >) to integer types

[guipublic]

Problem

Comparisons are enabled for TypeVariable so that we use these operators for instance to sort generic arrays, avoiding to re-implement the sort for many types.
The problem is that TypeVariable encompass types that cannot be compared, such as Field or structs.

Solution

Expose a numeric type so that users can write generic functions which will work with any numeric type.

Additional context

cf. discussion about sort function in std::lib in PR #754.