Fixing issues with XDRgen for Elixir

[elisa6]

After reviewing the next files .x from Stellar Core:

• Stellar SCP
• Stellar internal
• Stellar types

We need to fix the next issues:

• Not generate duplicate files
• The arms for a union need to be a map when the arms are integers
• Not repeat alias and types in unions
• The max length for the variable lists must be included in the module name and file name
• Include the Fixed prefix when is a fixed list
• Fix module name from Opaque types