flambda-backend: Allow Maybe_null jkinds in more places (#2747)

* Tests

* `Or_null` sort variables

* `Value_or_null` creation reasons

* `Any_non_null` creation reasons

* Accept or-null tests

* Change comment

* Hide nullability in jkinds if layouts is not at level alpha

* Fix printing error

* Improve printing; comments; formatting

* Split basics tests with and without alpha

* lessen diff

* fixes

* Fix rebase

* Promote test

* format

* fix

* promote tests

* Document hiding/substitutions

* `concrete_default` -> `concrete_legacy`

---------

Co-authored-by: Diana Kalinichenko <dkalinichenko@janestreet.com>

Author: dkalinichenko-js

jane/doc/extensions/unboxed-types/index.md

@@ -36,6 +36,14 @@ by a *type*. There is a small fixed set of layouts:
 * `any` is a layout that is the superlayout of all other layouts.  It doesn't correspond
   to a specific runtime representation. More information [below](#the-any-layout).
 
+* `value_or_null` is a superlayout of `value` including normal OCaml values
+  and null pointers. Unless `-extension-universe alpha` is set, it is displayed
+  as `value` and can't be used in jkind annotations.
+* `any_non_null` is a sublayout of `any` forbidding null pointers. Unless
+  `-extension-universe alpha` is set, it is displayed as `any`.
+  Additionally, `any` jkind annotations are interpreted as `any_non_null` for
+  backwards compatibility for definitions using arrays.
+
 Over time, we'll be adding more layouts here.
 
 # Layout annotation
@@ -353,19 +361,19 @@ orders:
 
   * Records
   * Constructors
-  
+
 Unboxed numbers can't be put in these structures:
 
   * Constructors with inline record fields
   * Exceptions
   * Extensible variant constructors
   * Top-level fields of modules
   * Tuples
-  
+
 There aren't fundamental issues with the structures that lack support. They will
 just take some work to implement.
 
-Here's an example of a record with an unboxed field. We call such a record 
+Here's an example of a record with an unboxed field. We call such a record
 a "mixed record".
 
 ```ocaml
@@ -429,7 +437,7 @@ These operations aren't supported:
   * polymorphic comparison and equality
   * polymorphic hash
   * marshaling
-  
+
 These operations raise an exception at runtime, similar to how polymorphic
 comparison raises when called on a function.
 
@@ -453,7 +461,7 @@ scanned by the garbage collector.
 [^can-or-must]: "Can-or-must" is a bit of a mouthful, but it captures the right nuance. Pointer values *must* be scanned, unboxed number fields *must* be skipped, and immediate values *can* be scanned or skipped.
 
 The ordering constraint on structure fields is a reflection of the same
-ordering restriction in the runtime representation. 
+ordering restriction in the runtime representation.
 
 ## C bindings for mixed blocks
 

testsuite/tests/typing-layouts-arrays/basics.ml

@@ -2,9 +2,6 @@
  include stdlib_upstream_compatible;
  flambda2;
  {
-   flags = "-extension layouts_alpha -extension small_numbers";
-   expect;
- }{
    flags = "-extension layouts_beta -extension small_numbers";
    expect;
  }{