fix string concat

src/ndspkg/types/ndstring.nim

@@ -89,7 +89,8 @@ proc `$`*(ndStr: NdString): string =
 
 proc `&`*(left, right: NdString): NdString =
   # TODO optimize this later when strings will be benchmarked
-  newString($left & $right)
+  let combined: string = $left & $right
+  newString(combined)
 
 proc getLength*(ndStr: NdString): int =
   ndStr.len.int

src/ndspkg/types/value.nim

@@ -207,7 +207,7 @@ proc add*(val: var NdValue, right: NdValue): NatReturn {.inline.} =
   if val.isFloat() and right.isFloat():
     val = fromFloat(val.asFloat() + right.asFloat())
   elif val.isString() and right.isString():
-    val = fromNdString(val.asString() & val.asString())
+    val = fromNdString(val.asString() & right.asString())
   else:
     return natError(&"Attempt to add types {val.friendlyType()} and {right.friendlyType()}.")
   return natOk