implement the 'bind' statement for generics, it was an oversight that this was never implemented (#14584)

tests/lookups/tbind_for_generics.nim

@@ -0,0 +1,17 @@
+discard """
+  errormsg: "type mismatch: got <Foo, Foo>"
+  line: 8
+"""
+proc g[T](x: T) =
+  bind `+`
+  # because we bind `+` here, we must not find the `+` for 'Foo' below:
+  echo x + x
+
+type
+  Foo = object
+    a: int
+
+proc `+`(a, b: Foo): Foo = Foo(a: a.a+b.a)
+
+g(3)
+g(Foo(a: 8))