core 109.21.00-rc1

# 109.21.00

## async_unix

- Added `Unix.remove`.

## core

- Massively improved the signatures of `Map` and `Set`, both for
  readability and ocamldoc, as well as improved type error messages.

  For instance the type of `Int.Set.singleton` was:

  ```ocaml
  ('a, 'comparator, 'a Core.Std.Int.Set.elt_ -> ('a, 'comparator) Core.Std.Int.Set.t_) Core.Core_set_intf.without_comparator
  ```

  Now it is simply:

  ```ocaml
  int -> Int.Set.t
  ```
- Added an optional argument to `Command.run` that can be used to
  specify default flags from a user config file.

  The optional argument can extend the command line based on the path
  to the command.
- Rename module `Weekday` as `Day_of_week`.

  The name `Weekday` conflicted with ordinary usage of "weekday" to
  mean Monday through Friday.
- Changed `sexp_of_t` for `{Month,Ofday,Time,Time.Span}.{Set,Map}` to
  use the nice sexp format of the underlying atomic type.

  Previously, the converter had used thes raw type (`float`, `int`,
  etc.).  `t_of_sexp` still accepts both formats; we will remove the
  ability to accept the raw format in the distant future.

  This output-format change was planned when we originally in 108.06b
  improved those `t_of_sexp` functions to accept both formats.
- Added `Unix.remove`.
- Removed some `IFDEF`'s connected to OCaml <4 support.

## zero

- Added module `Flat_tuple_array` for tuples packed into an array.
- Added function `Pool.length : _ t -> int`, which returns the number
  of live tuples in a pool.