gh-36173: Fix quadratic memory of berlekamp_massey
Fix #36172.
Reduced berlekamp_massey memory consumption by replacing a dictionary
with temporary variables. The memory consumption of the line highlighted
below reduced from 42MB to 4MB (probably inaccurate, but significant
enough).
```python
from memory_profiler import profile
from sage.matrix.berlekamp_massey import berlekamp_massey
@profile
def gen_data():
p = random_prime(2**64)
ls = [GF(p).random_element() for _ in range(20000)]
berlekamp_massey(ls); # <--- this line
gen_data()
```
I am not sure if I have to include extra doctests or not, or how to test
memory consumptions, since the time complexity is also O(n^2).
### :memo: Checklist
- [x] The title is concise, informative, and self-explanatory.
- [x] The description explains in detail what this PR is about.
- [x] I have linked a relevant issue or discussion.
- [x] I have created tests covering the changes.
- [x] I have updated the documentation accordingly.
URL: https://github.com/sagemath/sage/pull/36173
Reported by: grhkm21
Reviewer(s): grhkm21, Kwankyu Lee
