Restore the Nim's 0.14 proper handling of generic aliases

A more efficient implementation is possible by restoring the old
lifting ot tyGenericInvocation to tyGenericInst in liftTypeParam,
but this fix will suffice for now.

fixes #5087
fixes #5602
fixes #5641
fixes #5570

tests/generics/tmapping_generic_alias.nim

@@ -0,0 +1,28 @@
+discard """
+output: '''type(c) = GenAlias[system.int]
+T = int
+seq[int]
+'''
+"""
+
+import typetraits
+
+type
+  Gen[T] = object
+    x: T
+
+  GenAlias[T] = Gen[seq[T]]
+
+proc f1[T](x: Gen[T]) =
+  echo T.name
+
+proc f2[T](x: GenAlias[T]) =
+  echo "type(c) = ", type(x).name
+  echo "T = ", T.name
+  f1 x
+
+let
+  y = Gen[seq[int]](x: @[10])
+
+f2 y
+