A budget stops an expansion and a linear solve over polynomials

[Rafael-SOWNet]

Part of #718.

A computation under a cancellation token -- the corpus harness's budget, or a caller's timeout -- ran on past the cancellation inside an expansion and inside the linear solve over polynomials that partial fractions makes, since neither looked at the token. On sixty of the problems that ran past the 5-second budget in the corpus run on master cb8e133d, the first sixty of families 1, 3, 4 and 6, run one after another in one process per build at that budget, each on a thread of its own and a problem that does not stop left running beside the next, under a 2 GB heap:

	master 7075a747	this
stopped more than half a second after the budget	16	2
more than five seconds after it	8	0
still running a minute after it	7	0
out of memory	1	0
the whole run	733 s	282 s

The latest any of this column stopped is 6.7 s. Sampled past the cancellation on master, four of the seven that ran on for a minute were in the linear solve over polynomials, two of them, in an expansion, one, and evaluating at pinned symbols what partial fractions had built, one.

What changes. MultithreadingFunctional.ExitIfCancelled, which forty other places in the library call -- the simplifier, the polynomial GCD and factorisation, the solvers, the limits -- is called once per expansion of a product or a power, TreeAnalyzer.SmartExpandOver, and once per row of the elimination in PartialFractions.TrySolveOverPolynomials. MaxExpansionTermCount bounds the terms of one expansion, not the time the expansions of a computation take between them. With no token set the call does nothing, and no answer differs, so there is no entry in BREAKING-CHANGES.md, as there was none for #870; a caller whose token is cancelled gets the cancellation from an expansion now, as from the other steps.

Tests: ExpansionCancelTest: an expansion of (a + b + c)^5 (d + e)^3 under a token cancelled before it starts throws OperationCanceledException, and on master it completes; one that is not cancelled completes.

The suite passes on the commit measured, 93dcd0ac, 14,782 tests with 13 skipped. On the merge with master 3f744439, 14f4da3d, the multithreading tests, #1743's and the corpus gate pass, 115 of them. The allocation gate was not run here; the check is one read of an AsyncLocal per call and allocates nothing, and CI runs the gate.

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