Pattern matching notation for monadic binds

See merge request robbertkrebbers/coq-stdpp!18

This gets rid of the old hack to have specific notations for pairs
up to a fixed arity, and moreover allows to do fancy things like:

```
Record test := Test { t1 : nat; t2 : nat }.

Definition foo (x : option test) : option nat :=
  ''(Test a1 a2) ← x;
  Some a1.
```

This one works for setoid rewriting under binders.

Factor out solve_proper_prepare

See merge request robbertkrebbers/coq-stdpp!19

- Name all variables that we refer to.
- Put types in definitions.

Provide an Infinite typeclass and a generic implementation of Fresh.

See merge request robbertkrebbers/coq-stdpp!13

Make `fmap` left associative.

See merge request robbertkrebbers/coq-stdpp!16

This follows the associativity in Haskell. So, something like

  f <$> g <$> h

Is now parsed as:

  (f <$> g) <$> h

Since the functor is a generalized form of function application, this also now
also corresponds with the associativity of function application, which is also
left associative.

This is similar to `f_equal/=`.

Use `stdpp_scope` for all notations.

See merge request robbertkrebbers/coq-stdpp!17

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