Also make the instances non-global, to prevent multiple instance
problems.

We prove that various types are infinite, notably:
- nat, N, positive and Z;
- string (using pretty-printing of nat);
- option, with an infinite element type;
- list, with an inhabited element type.

Furthermore, we instantiate Fresh for strings.

This implements a simple linear search for fresh elements.

This generalizes Fix_unfold to a setoid setting. In particular,
we can use this to unfold multi-argument fixpoints without
requiring functional extensionality.

Infinity is described by having an injection from nat.

Provide a pretty-printer for [nat].

See merge request robbertkrebbers/coq-stdpp!15

Minor documentation fixes

See merge request robbertkrebbers/coq-stdpp!14

The documentation for some typeclasses used the wrong
names for these typeclasses.

Notation for disjointness: replace ⊥ with ##, so that ⊥ can be used for bottom.

See merge request robbertkrebbers/coq-stdpp!12

This addresses some concerns in !5.

Add monadic `;;` and change level of the do-notation to 100

See merge request robbertkrebbers/coq-stdpp!10

This way, we will be compabile with Iris's heap_lang, which puts ;;
at level 100.

Add more lemmas for gmap uncurry

See merge request robbertkrebbers/coq-stdpp!9

Add more properties of intersection_with for fin_maps

See merge request robbertkrebbers/coq-stdpp!7