macros: Introduce sameType(a, b) for node types

Previously introduced node comparison `==` was working somehow wrong on nodes
returned from getType(a), comparing just ids of the symbols.

Recently introduced `==` change 47dce26886
started comparing symbol nodes pointer-wise, thus strictly. Since getType(a)
always creates new symbol pointing to the type, comparing two such nodes using
`==` always returns false, even they point to the same type.

That is why we need a new sameType macro to be able to tell if these nodes
point to the same type.

lib/core/macros.nim

@@ -144,6 +144,10 @@ proc `==`*(a, b: NimIdent): bool {.magic: "EqIdent", noSideEffect.}
 proc `==`*(a, b: NimNode): bool {.magic: "EqNimrodNode", noSideEffect.}
   ## compares two Nim nodes
 
+proc sameType*(a, b: NimNode): bool {.magic: "SameNodeType", noSideEffect.}
+  ## compares two Nim nodes' types. Return true if the types are the same,
+  ## eg. true when comparing alias with original type.
+
 proc len*(n: NimNode): int {.magic: "NLen", noSideEffect.}
   ## returns the number of children of `n`.