flint: perf: use mpoly for sparse univariate computations

[jodavies]

This improves performance for things like "gcd_(1-x^20,1-x^1000000)" since FLINT poly has a dense representation whereas mpoly is sparse. "Real life" benchmarks such as forcer or minceex are unchanged, they have dense polynomials.

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Any thoughts on this? I have determined the threshold "experimentally" by benchmarking gcd_(1-x^20,1-x^N) for a range of N. Of course different computations probably have a different threshold... this kind of heuristic usually has tricky cases.

[coveralls]

coverage: 56.759% (-0.02%) from 56.779%
when pulling b79cc4c on jodavies:flint-sparse
into 2aba64b on form-dev:master.

[tueda]

I'm not sure if users should be able to choose the threshold, but in principle, it could be a setup parameter that they can adjust if needed.

[jodavies]

Right, this is an option.

For the record, forcer and minceex are ~25% slower if you just force the use of mpoly always.

[jodavies]

Updated, the term counting was not correct in fact.

I ran a few benchmarks also for mul_ and div_, the optimal threshold for those is not exactly the same (for the tested polynomials) as for gcd_ but the value I chose before is broadly OK.