Add some enhancements to jsonutils.nim (#15133)

* Add some enhancements to `jsonutils.nim`

* Use `jsonutils.nim` hookable API to add a possibility to deserialize
  JSON arrays directly to `HashSet` and `OrderedSet` types and
  respectively to serialize those types to JSON arrays.

* Also add a possibility to deserialize JSON `null` objects to Nim
  option objects and respectively to serialize Nim option object to JSON
  object if some or to JSON `null` object if none.

* Move serialization/deserialization functionality for `Table` and
  `OrderedTable` types from `jsonutils.nim` to `tables.nim` via the
  hookable API.

* Add object `jsonutils.Joptions` and parameter from its type to
  `jsonutils.fromJson` procedure to control whether to allow
  deserializing JSON objects to Nim objects when the JSON has some
  extra or missing keys.

* Add unit tests for the added functionalities to `tjsonutils.nim`.

* improve fromJsonFields

* Add changelog entry for the jsonutils enhancements

* Add TODO in `jsonutils.nim`

* Added an entry to "Future directions" section in `jsonutils.nim` as
  suggestion for future support of serialization and de-serialization of
  nested variant objects.

* Added currently disabled test case in `tjsonutils.nim` for testing
  serialization and de-serialization of nested variant objects.

* Move JSON hooks to `jsonutils.nim`

Move `fromJsonHook` and `toJsonHook` procedures for different types to
`jsonutils.nim` module to avoid a dependency of collections modules to
the `json.nim` module.

The hooks are removed from the following modules:

  * `tables.nim`

  * `sets.nim`

  * `options.nim`

  * `strtabs.nim`

* Add some tests about `StringTableRef`

Add tests for `StringTableRef`'s `fromJsonHook` and `toJsonHook` to
`tjsonutils.nim`.

* Disable a warning in `jsonutils.nim`

Mark `fun` template in `jsonutils` module with `{.used.}` pragma in
order to disable `[XDeclaredButNotUsed]` hint. The template is actually
used by the `initCaseObject` macro in the same module.

Co-authored-by: Timothee Cour <timothee.cour2@gmail.com>

lib/pure/options.nim

@@ -372,7 +372,6 @@ proc unsafeGet*[T](self: Option[T]): lent T {.inline.}=
   assert self.isSome
   result = self.val
 
-
 when isMainModule:
   import unittest, sequtils