add isNamedTuple; make $(1, 2) be (1, 2) instead of (Field0: 1, Field1: 2) which leaked implementation detail (#10070)

* add `isNamedTuple`; make $(1, 2) be (1, 2) instead of leaking implementation detail (Field0: 1, Field1: 2)
  fixes this: #8670 (comment) /cc @alehander42 @Vindaar @mratsim

* Note: isNamedTuple is useful in other places, eg #10010 (comment)

* move isNamedTuple to helpers.nim to avoid exposing new symbol to system.nim

* remove workaround in tests/vm/tissues.nim failing test now that #10218 was makes it work

lib/system/helpers.nim

@@ -6,6 +6,23 @@
 proc lineInfoToString(file: string, line, column: int): string =
   file & "(" & $line & ", " & $column & ")"
 
-proc `$`(info: type(instantiationInfo(0))): string =
+type InstantiationInfo = tuple[filename: string, line: int, column: int]
+
+proc `$`(info: InstantiationInfo): string =
   # The +1 is needed here
+  # instead of overriding `$` (and changing its meaning), consider explicit name.
   lineInfoToString(info.fileName, info.line, info.column+1)
+
+proc isNamedTuple(T: type): bool =
+  ## return true for named tuples, false for any other type.
+  when T isnot tuple: result = false
+  else:
+    var t: T
+    for name, _ in t.fieldPairs:
+      when name == "Field0":
+        return compiles(t.Field0)
+      else:
+        return true
+    # empty tuple should be un-named,
+    # see https://github.com/nim-lang/Nim/issues/8861#issue-356631191
+    return false

lib/system/helpers2.nim

@@ -1,3 +1,5 @@
+# imported by other modules, unlike helpers.nim which is included
+
 template formatErrorIndexBound*[T](i, a, b: T): string =
   "index out of bounds: (a:" & $a & ") <= (i:" & $i & ") <= (b:" & $b & ") "