implements overloadable enum values; WIP (#18470)

* implements overloadable enum values
* simpler code

tests/enum/toverloadable_enums.nim

@@ -0,0 +1,48 @@
+discard """
+  output: '''B
+0'''
+joinable: false
+"""
+
+{.experimental: "overloadableEnums".}
+
+type
+  E1 = enum
+    value1,
+    value2
+  E2 = enum
+    value1,
+    value2 = 4
+
+const
+  Lookuptable = [
+    E1.value1: "1",
+    value2: "2"
+  ]
+
+when false:
+  const
+    Lookuptable: array[E1, string] = [
+      value1: "1",
+      value2: "2"
+    ]
+
+
+proc p(e: E1): int =
+  # test that the 'case' statement is smart enough:
+  case e
+  of value1: echo "A"
+  of value2: echo "B"
+
+
+let v = p value2 # ERROR: ambiguous!
+# (value2|value2)  nkClosedSymChoice -> nkSym
+
+proc x(p: int) = discard
+proc x(p: string) = discard
+
+proc takeCallback(param: proc(p: int)) = discard
+
+takeCallback x
+
+echo ord v

tests/errmsgs/t10251.nim

@@ -2,7 +2,6 @@ discard """
   action:reject
   cmd: "nim check $options $file"
   nimout: '''
-t10251.nim(15, 5) Error: redefinition of 'foo'; previous declaration here: t10251.nim(13, 5)
 t10251.nim(19, 23) Error: redefinition of 'goo1'; previous declaration here: t10251.nim(19, 11)
 '''
 """
@@ -14,6 +13,7 @@ type
   Enum2 = enum
     foo, bar, baz
 
+
 type
   Enum3 {.pure.} = enum # fixed (by accident?) in https://github.com/nim-lang/Nim/pull/18263
     goo0, goo1, goo2, goo1