Overhaul of the `Infinite`/`Fresh` infrastructure.
- The class `Infinite A` is now defined as having a function `fresh : list A → A`, that given a list `xs`, gives an element `x ∉ xs`. - For most types this `fresh` function has a sensible computable behavior, for example: + For numbers, it yields one added to the maximal element in `xs`. + For strings, it yields the first string representation of a number that is not in `xs`. - For any type `C` of finite sets with elements of infinite type `A`, we lift the fresh function to `C → A`. As a consequence: - It is now possible to pick fresh elements from _any_ finite set and from _any_ list with elements of an infinite type. Before it was only possible for specific finite sets, e.g. `gset`, `pset`, ... - It makes the code more uniform. There was a lot of overlap between having a `Fresh` and an `Infinite` instance. This got unified.
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