Fix duplicate type tags in the encoding

[jcp19]

As it stands, Gobra generates repeated occurrences of the following four functions, corresponding to the type tags of int32 and rune (its synonym), and int8 and bytes (also synonyms) (for some reason, this problem was never triggered in the CI before). This PR addresses that.

  function int32_Types(): Types
  unique function int32_Types_tag(): Int
  function byte_Types(): Types
  unique function byte_Types_tag(): Int