Implement conversion from laurent series to rational function field

[user202729]

As in the title. I think this behavior makes sense because QQ(RR(…)) currently uses .simplest_rational() instead of like exact value (like x / 2^53 for some integer x), so it makes sense for similar conversions to compute a good approximation too.

I cannot prove that this round-trips, however.

Note that currently conversion from power series QQ[[x]] to polynomial ring QQ[x] truncates, because of implementation detail this leads to conversion from QQ[[x]] to Frac(QQ[x]) also truncates. I think this is undesirable behavior because identical-looking elements in QQ[[x]] versus Frac(QQ[[x]]) = LaurentSeriesRing(QQ, "x") has different conversion behavior.

Elements which are already Laurent polynomial are preserved.

Side note: .one() is faster because it's cached

sage: R.<x> = QQ[]
sage: S = Frac(R)
sage: %timeit S.one()  # fast because it's cached
90.6 ns ± 1.27 ns per loop (mean ± std. dev. of 7 runs, 10,000,000 loops each)
sage: ring_one = S.ring().one()
sage: %timeit S._element_class(S, ring_one, ring_one, coerce=False, reduce=False)
3.42 μs ± 75 ns per loop (mean ± std. dev. of 7 runs, 100,000 loops each)

Maybe this doesn't need to be explicitly written in the documentation (users trying to do the conversion will just see the result)

There exists also :meth:`.PowerSeries_poly.pade` which should probably be mentioned in the documentation.

Reverse direction (that one is a morphism): #39365

Possible caveat:

sage: S(Frac(V)(1/(x+1))/(x^10))
1/(x^11 + x^10)
sage: S(Frac(V)(1/(x+1))/(x^30))
(-x^19 + x^18 - x^17 + x^16 - x^15 + x^14 - x^13 + x^12 - x^11 + x^10 - x^9 + x^8 - x^7 + x^6 - x^5 + x^4 - x^3 + x^2 - x + 1)/x^30

It could be argued the latter would be simpler as 1/(x^31+x^30), but then that way gives higher denominator degree.

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