enum: check enum value alias uniqueness when define enum type in sheet or proto file

[wenchy]

1. Problem in sheet

A sheet named ItemType (also the enum type name) in workbook:

Name	Alias
ITEM_TYPE_DIAMOND	Diamond
ITEM_TYPE_EQUIP	Diamond
ITEM_TYPE_BOX	Box

Note the enum value alias Diamond is duplicate.

2. Problem in proto file

enum ItemType {
  ITEM_TYPE_INVALID = 0;
  ITEM_TYPE_DIAMOND = 1 [(tableau.evalue).name = "Diamond"];
  ITEM_TYPE_EQUIP = 2 [(tableau.evalue).name = "Diamond"];
  ITEM_TYPE_BOX = 3 [(tableau.evalue).name = "Box"];
}