fixes [MPR#7868](https://caml.inria.fr/mantis/view.php?id=7868)

- inline Pervasives in Stdlib and re-add Pervasives as a deprecated
module that aliases all elements of Stdlib except the stdlib modules.

- remove special case for Stdlib.Pervasives in printtyp.ml

* add `Seq` module, expose iterator conversions in most containers

* small typo

* typo

* change order of arguments for `{Map,Set}.add_seq`

* watch for max string length in `Bytes.of_seq`

* wip: make it build again

* Fix dependency

Sys needs to be linked before Bytes in stdlib.

* Update threads/stdlib.ml

* Update stdlib_no_prefixed/.depend

* fix inconsistencies with label modules

* update testsuite to work with seq

* update change file

* small change in `Hashtbl.to_seq`, capturing only the underlying array

* add some documentation to seq.mli

* revert to good ol' module type names for hashtables

* fix test

* change style of comments in seq.mli

* follow some demands in review of GPR #1002

* some fixes for #1002

* add Seq-related functions to Ephemeron

* add some comments on `Hashtbl.of_seq`

* add more tests for `Hashtbl.{to,of}_seq`

* fix bug in `Ephemeron.to_seq`

* Update Changes

Finds the first/last binding where the key satisfies a monotonic
predicate.

Addresses #665, supersedes #868.
Thanks @alainfrisch for the idea and most of the implementation.

Provide an xxx_opt alternative for functions raising Not_found
and many instances of Failure/Invalid_arg.

The only exception is the rarely used Buffer.add_substitute, where
the [Not_found] can really be interpreted as an error condition.

Most new functions are implemented directly (instead of wrapping the
raising version).  This is for performance reasons and also to avoid
destroying the stacktrace (if the function is used in an exception
handler).  One could instead implement the raising versions on top of
the new functions, but there might be a small penalty.