Rename 'open' to 'include', then deprecate 'open'

[elpinal]

Currently, SATySFi has 3 forms of open:

1. open M: global open
2. open M in expr: local open
3. M.(expr): lightweight local open

Only (1) is related to this PR. The other 2 forms are irrelevant.

SATySFi's global open (hereafter, just call open) is similar to OCaml's include, not OCaml's open, because it re-exports the definitions in a module.

open / include in modules, wrap-up:

	Standard ML	OCaml	SATySFi (current)	SATySFi (ideal)
re-export	open	include	open	include
not re-export		open		open

In my experience with Standard ML, the absence of OCaml-style open is to some extent painful, even though Standard ML has local ... in ... end, which makes Standard ML-style open a bit more local.

Moreover, we cannot omit module prefixes in signatures (in a natural way) if OCaml-style open is not supported. Compare these (contrived) OCaml programs:

sig
  type s = int * Mod.t
  val v : Mod.t list -> Mod.u option
  val f : Mod.t -> s
end

sig
  open Mod
  type s = int * t
  val v : t list -> u option
  val f : t -> s
end

On the other hand, OCaml-style include is also useful, especially when combined with functors and to extend modules with additional definitions.

So I believe both open and include should be implemented in SATySFi. Since SATySFi has many similarities with OCaml, it would be appropriate to use the same keywords as OCaml. As a first step, this PR adds a keyword include as an alias to open. Furthermore, if a program contains (global) open, SATySFi warns about it, leading the programmer to refrain from using open.

Plan

I propose the following plan.

• Phase 1: Introduce include, and warn the use of open. (What this PR does.)
• Phase 2: Remove open.
• Phase 3: Introduce OCaml-style open.