gh-35455: Faster get_unsafe for NTL GF(p) polynomials
### :books: Description
Unlike similar Zmod(n) polynomials, the GF(p) NTL polynomials would call
the IntegerModRing constructor for each get_unsafe call resulting in
very slow polynomial evaluation.
Benchmarks:
```
p = next_prime(2**80)
pol = GF(p)["x"].random_element(80000)
# Before
# evaluate pol at a random GF(p) element: 260.6ms
# evaluate pol at a random GF(p^2) element: 790.1ms
# evaluate pol at a random GF(p^5) element: 1155.8ms
# After
# evaluate pol at a random GF(p) element: 66.8ms
# evaluate pol at a random GF(p^2) element: 541.2ms
# evaluate pol at a random GF(p^5) element: 889.9ms
```
### :memo: Checklist
- [x] The title is concise, informative, and self-explanatory.
- [x] 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 accordingly.
URL: https://github.com/sagemath/sage/pull/35455
Reported by: Rémy Oudompheng
Reviewer(s): Marc Mezzarobba, Rémy Oudompheng
