Macros: Introduce getType(t: typedesc): NimNode

Since typedesc are exception in macros and they are not implicitly converted to
NimNode on macro call, we need some means to perform such conversion on demand.

Fortunately it is as simple as declaring new magic "NGetType" proc with
typedesc parameter.

NOTE: Keeping actual macro exceptional behavior for typedesc is important,
since it allows passing typedesc macro parameter to other procs or macros
expecting type parameter. If typedesc parameter was implicitly converted, then
we would lost this ability.

tests/macros/tgettype.nim

@@ -0,0 +1,20 @@
+discard """
+msg: '''ObjectTy(Sym(Model), RecList(Sym(name), Sym(password)))
+BracketExpr(Sym(typeDesc), Sym(User))'''
+"""
+import strutils, macros
+
+type
+  Model = object of RootObj
+  User = object of Model
+    name : string
+    password : string
+
+macro testUser: expr =
+  return newLit(User.getType.lispRepr)
+
+macro testGeneric(T: typedesc[Model]): expr =
+  return newLit(T.getType.lispRepr)
+
+echo testUser
+echo User.testGeneric