clarify the meaning of the 'auto' metatype; 'auto' is now bind-multiple; fixes #3224

tests/types/tauto_excessive.nim

@@ -0,0 +1,20 @@
+discard """
+  output: '''10
+10.0
+1.0hiho'''
+"""
+
+# bug #3224
+proc f(x: auto): auto =
+  result = $(x+10)
+
+proc f(x, y: auto): auto =
+  result = $(x+y)
+
+
+echo f(0)     # prints 10
+echo f(0.0)  # prints 10.0
+
+proc `+`(a, b: string): string = a & b
+
+echo f(0.7, 0.3), f("hi", "ho")