gh-35307: Use StringIO to format polynomials
This patch modifies the repr()/str() method of polynomials to use a
StringIO buffer instead of `s += ...` to build the result string. It is
known that repeatedly calling `+=` has quadratic complexity for large
Python strings.
Currently calling `str()` on a high degree polynomial takes a very long
amount of time and it may happen accidentally (a script was stuck during
2 hours trying to format a degree 300000 polynomial for an exception
message).
```
sage: p = random_prime(2**96)
sage: K = GF(p**24, 'a')
sage: pol = K["x"].random_element(30000)
# Before patch
sage: %time _ = str(pol)
CPU times: user 2min 22s, sys: 1min 43s, total: 4min 5s
Wall time: 4min 5s
# After patch
sage: %time _ = str(pol)
CPU times: user 692 ms, sys: 21 ms, total: 713 ms
Wall time: 713 ms
```
### 📝 Checklist
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### ⌛ Dependencies
None.
URL: https://github.com/sagemath/sage/pull/35307
Reported by: Rémy Oudompheng
Reviewer(s): Lorenz Panny
