Signed primitive integers defined on top of the existing unsigned ones
with two's complement.

The module Sint63 includes the theory of signed primitive integers that
differs from the unsigned case.

Additions to the kernel:
  les (signed <=), lts (signed <), compares (signed compare),
  divs (signed division), rems (signed remainder),
  asr (arithmetic shift right)
(The s suffix is not used when importing the Sint63 module.)

The printing and parsing of primitive ints was updated and the
int63_syntax_plugin was removed (we use Number Notation instead).

A primitive int is parsed / printed as unsigned or signed depending on
the scope. In the default (Set Printing All) case, it is printed in
hexadecimal.

Ack-by: SkySkimmer
Ack-by: ppedrot
Reviewed-by: vbgl

Following a request from Pierre-Marie Pédrot in #13258

Reviewed-by: erikmd
Reviewed-by: herbelin

We also put them in a module, so users can `Require Int63. Import
Int63.Int63Notations` without needing to unqualify the primitives.

In particular, we change
- `a \% m` into `a mod m` to correspond with the notation in ZArith
- `m == n` into `m =? n` to correspond with the eqb notations elsewhere
- `m < n` into `m <? n` to correspond with the ltb notations elsewhere
- `m <= n` into `m <=? n` to correspond with the leb notations elsewhere
- `m ≤ n` into `m ≤? n` for consistency with the non-unicode notation

The old notations are still accessible as deprecated notations.

Fixes #12454

This is a companion to #12479 as per
https://github.com/coq/coq/pull/12479#issuecomment-641336039 that
changes some of the PrimFloat notations:
- `m == n` into `m =? n` to correspond with the eqb notations elsewhere
- `m < n` into `m <? n` to correspond with the ltb notations elsewhere
- `m <= n` into `m <=? n` to correspond with the leb notations elsewhere

We also put them in a module, so users can `Require PrimFloat. Import
PrimFloat.PrimFloatNotations` without needing to unqualify the
primitives.

Fixes the part of #12454 about floats

Persistent arrays expose a functional interface but are implemented
using an imperative data structure. The OCaml implementation is based on
Jean-Christophe Filliâtre's.

Co-authored-by: Benjamin Grégoire <Benjamin.Gregoire@inria.fr>
Co-authored-by: Gaëtan Gilbert <gaetan.gilbert@skyskimmer.net>