Fix fat pointers, object copying, magic double evals on JS (#9411) [backport]

* Add a test for issue #9389

* Fixes #9389.

* Make object contructors copy objects properly by checking whether the expressions passed to them don't need to be copied.
* Make mArrToSeq implementation actually check if a copy needs to be made.

* Avoid unnecessary copy in mChr impl

* Assume set constructor elements need no copy

* Add a test for issue #9410

* Add a test

* fix passing fat pointers (#9410)

* Enhance tests

* More tests and fixes

* Add more (failing) tests [ci skip]

* Added equality operator for fat pointers, more tests and fixes

* Fix printing uninitialized strings

* Fix mInc, mDec double eval, add more tests

* Tests

* Refactored, fixed multiple evals, revamped the tests, added missing ops

* Fix ups

* Fix #9643 and #9644

* add pointer normalization

lib/system/jssys.nim

@@ -519,8 +519,11 @@ proc nimCopyAux(dest, src: JSRef, n: ptr TNimNode) {.compilerproc.} =
       `dest`[`n`.offset] = nimCopy(`dest`[`n`.offset], `src`[`n`.offset], `n`.typ);
     """
   of nkList:
-    for i in 0..n.len-1:
-      nimCopyAux(dest, src, n.sons[i])
+    asm """
+    for (var i = 0; i < `n`.sons.length; i++) {
+      nimCopyAux(`dest`, `src`, `n`.sons[i]);
+    }
+    """
   of nkCase:
     asm """
       `dest`[`n`.offset] = nimCopy(`dest`[`n`.offset], `src`[`n`.offset], `n`.typ);

lib/system/reprjs.nim

@@ -64,7 +64,9 @@ proc reprStrAux(result: var string, s: cstring, len: int) =
 
 proc reprStr(s: string): string {.compilerRtl.} =
   result = ""
-  if cast[pointer](s).isNil:
+  var sIsNil = false
+  asm """`sIsNil` = `s` === null"""
+  if sIsNil: # cast[pointer](s).isNil:
     # Handle nil strings here because they don't have a length field in js
     # TODO: check for null/undefined before generating call to length in js?
     # Also: c backend repr of a nil string is <pointer>"", but repr of an