Fixes for new runtime (#7037)

lib/core/seqs.nim

@@ -7,7 +7,7 @@
 #    distribution, for details about the copyright.
 #
 
-import allocators
+import allocators, typetraits
 
 ## Default seq implementation used by Nim's core.
 type
@@ -115,3 +115,25 @@ proc `@`*[T](elems: openArray[T]): seq[T] =
       result.data[i] = elems[i]
 
 proc len*[T](x: seq[T]): int {.inline.} = x.len
+
+proc `$`*[T](x: seq[T]): string =
+  result = "@["
+  var firstElement = true
+  for i in 0..<x.len:
+    let 
+      value = x.data[i]
+    if firstElement:
+      firstElement = false
+    else:
+      result.add(", ")
+
+    when compiles(value.isNil):
+      # this branch should not be necessary
+      if value.isNil:
+        result.add "nil"
+      else:
+        result.addQuoted(value)
+    else:
+      result.addQuoted(value)
+
+  result.add("]")