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/strtabs.nim

@@ -89,6 +89,7 @@ const
   growthFactor = 2
   startSize = 64
 
+proc mode*(t: StringTableRef): StringTableMode {.inline.} = t.mode
 
 iterator pairs*(t: StringTableRef): tuple[key, value: string] =
   ## Iterates over every `(key, value)` pair in the table `t`.
@@ -422,25 +423,6 @@ proc `%`*(f: string, t: StringTableRef, flags: set[FormatFlag] = {}): string {.
       add(result, f[i])
       inc(i)
 
-since (1,3,5):
-  proc fromJsonHook*[T](a: var StringTableRef, b: T) =
-    ## for json.fromJson
-    mixin jsonTo
-    var mode = jsonTo(b["mode"], StringTableMode)
-    a = newStringTable(mode)
-    let b2 = b["table"]
-    for k,v in b2: a[k] = jsonTo(v, string)
-
-  proc toJsonHook*[](a: StringTableRef): auto =
-    ## for json.toJson
-    mixin newJObject
-    mixin toJson
-    result = newJObject()
-    result["mode"] = toJson($a.mode)
-    let t = newJObject()
-    for k,v in a: t[k] = toJson(v)
-    result["table"] = t
-
 when isMainModule:
   var x = {"k": "v", "11": "22", "565": "67"}.newStringTable
   assert x["k"] == "v"