Create a stopgap warning

[roed]

Sometimes issues can produce mathematically incorrect results, and we should warn users of the problem. If the issue is fixed promptly then nothing more needs to be done, but sometimes tickets remain unresolved for months or years (c.f. #11358). This ticket adds a function that makes it easy to create a second ticket warning users of the problem while it's being resolved.

On trac, this functionality will be supported by the Stopgaps field. Create a new ticket that uses the stopgap function and include a link to it in the Stopgaps field in the original ticket.

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Apply attachment:trac_12691-faster.patch​

[jen]

Looks good.

[jhpalmieri]

See #12702 for a followup.

[was]

I think this code is too slow. We want it to be highly optimized so as to not have much of a performance penalty for introducing a stopgap. I'm going to attach a different patch that implements stopgaps, and is an *order of magnitude* faster than this code:

sage: timeit("sage.misc.misc.stopgap('abc', 123)", number=10^5)
100000 loops, best of 3: 2.49 µs per loop
sage: import sage.misc.stopgap
sage: timeit("sage.misc.stopgap.stopgap('abc', 123)", number=10^5)
100000 loops, best of 3: 388 ns per loop
sage: 2.49 / .388
6.41752577319588

[ohanar]

everything looks good to me and works how I expect it to, positive review!