This reverts part of commit 54954f55.

Make `gset` a `Definition` instead of `Notation`.

See merge request iris/stdpp!59

This fixes an issue in orc11.

Overhaul of the `Infinite`/`Fresh` infrastructure

See merge request !58

- 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.

Notion of (in)finite predicates

See merge request !56

Additionally lemmas for insert, nth, take, and list_find

See merge request !55

Rename multiset "union" into "disjoint union"

Closes #13

See merge request !57

Also, use the union name/class/symbol for what's usually the union, and
define the intersection on multisets.

fix or silence Coq 8.10 warnings

See merge request !52

New `seq` operation on maps + consistency tweaks for `seq` operation on sets

See merge request !44