feat: disallow re-using token names and symbols within a pool

[rflechtner]

fixes https://github.com/KILTprotocol/ticket/issues/3803

Disallows re-using the same token symbol or name. Empty names and symbols are always allowed.

Checklist:

• I have verified that the code works
  • No panics! (checked arithmetic ops, no indexing array[3] use get(3), ...)
• I have verified that the code is easy to understand
  • If not, I have left a well-balanced amount of inline comments
• I have left the code in a better state
• I have documented the changes (where applicable)
  • Either PR or Ticket to update the Docs
  • Link the PR/Ticket here

[rflechtner]

Failing tests suggest that HashSet is not available to be used in the runtime, so I replaced it with BTreeSet