Faster cardinality and repr implementation of Set_object_enumerated

[user202729]

Otherwise, you try to print out a Set(GF(998244353)) and it eats all your computer's memory

compare with

sage: repr(matrix.zero(100))
'100 x 100 dense matrix over Integer Ring'
sage: str(matrix.zero(100))[:50]
'[0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0'
sage: repr(matrix.zero(3))
'[0 0 0]\n[0 0 0]\n[0 0 0]'
sage: str(matrix.zero(3))
'[0 0 0]\n[0 0 0]\n[0 0 0]'

📝 Checklist

• The title is concise and informative.
• The description explains in detail what this PR is about.
• I have linked a relevant issue or discussion.
• I have created tests covering the changes.
• I have updated the documentation and checked the documentation preview.

⌛ Dependencies