perf: add a flag to turn off isinstance in TypeTracerArray._new

[jpivarski]

The runtime type-checks in TypeTracerArray._new can now be turned off by setting

awkward.typetracer.TypeTracerArray.runtime_typechecks = False

which is a developer-public (but user-hidden) global interface.

[jpivarski]

I forgot to say, cc: @martindurant and @lgray.

[martindurant]

Please ping me when it's in. In dak, I can make the context manager right away, and it will have no downside: before this change it will simply have no effect.

[jpivarski]

Does this really need to be a context manager in dask-awkward, @martindurant? Because it is now a public (though hidden) interface and Coffea can just

awkward.typetracer.TypeTracerArray.runtime_typechecks = False

Being a context manager suggests that one would want to turn it off only for limited times, but I think Coffea wants to turn it off globally and only revisit that if there are any errors, right, @lgray?

[martindurant]

With this, you have both possibilities. I would say that, generally, a library shouldn't change any state in another library except in limited scopes.

[jpivarski]

Oh, that's right—Coffea might not be the only user of Awkward in a Python process. I was thinking of this as a global, set-it-once thing, like vector.register_awkward(), but that only adds behaviors to ak.behavior (which, in principle, might overshadow another library's, but in practice likely doesn't). This setting doesn't append state, it replaces state, and I can see how that would go wrong.