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

tests/metatype/tmetatype_issues.nim

@@ -1,7 +1,7 @@
 discard """
 output:'''
 void
-(Field0: "string", Field1: "string")
+("string", "string")
 1 mod 7
 @[2, 2, 2, 2, 2]
 impl 2 called

tests/metatype/ttypetraits2.nim

@@ -1,3 +1,18 @@
 # todo: merge with $nimc_D/tests/metatype/ttypetraits.nim (currently disabled)
 
 from typetraits import `$` # checks fix for https://github.com/c-blake/cligen/issues/84
+
+import typetraits
+
+block: # isNamedTuple
+  type Foo1 = (a:1,).type
+  type Foo2 = (Field0:1,).type
+  type Foo3 = ().type
+  type Foo4 = object
+
+  doAssert (a:1,).type.isNamedTuple
+  doAssert Foo1.isNamedTuple
+  doAssert Foo2.isNamedTuple
+  doAssert not Foo3.isNamedTuple
+  doAssert not Foo4.isNamedTuple
+  doAssert not (1,).type.isNamedTuple