fixes #8916 by fixing typeinfo and marshal. (#9341)

* fixes #8916 by removing `tyString`, `tySeq`, mod. marshal, typeinfo

Need to check in `typeinfo` for nil of the underlying pointer.
In marshal don't have to check for nil of seq anymore.

* remove reference to string, sequence in `isNil` doc string

tests/stdlib/tmarshal.nim

@@ -105,3 +105,16 @@ r = to[Something](data2)
 
 echo r.x, " ", r.y
 
+
+type
+  Foo = object
+    a1: string
+    a2: string
+    a3: seq[string]
+    a4: seq[int]
+    a5: seq[int]
+    a6: seq[int]
+var foo = Foo(a2: "", a4: @[], a6: @[1])
+foo.a6.setLen 0
+doAssert $$foo == """{"a1": "", "a2": "", "a3": [], "a4": [], "a5": [], "a6": []}"""
+testit(foo)