toJson, jsonTo, json (de)serialization for custom types; remove dependency on strtabs thanks to a hooking mechanism (#14563)

* json custom serialization; application for strtabs
* serialize using nesting
* make toJson more feature complete
* add since
* Revert "Improve JSON serialisation of strtabs (#14549)"

This reverts commit 7cb4ef26ad.

* better approach via mixin
* toJson, jsonTo
* fix test
* address comments
* move to jsonutils
* doc
* cleanups
* also test for js
* also test for vm

lib/pure/strtabs.nim

@@ -71,7 +71,7 @@ type
   StringTableObj* = object of RootObj
     counter: int
     data: KeyValuePairSeq
-    mode*: StringTableMode
+    mode: StringTableMode
 
   StringTableRef* = ref StringTableObj
 
@@ -419,6 +419,24 @@ 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