Add a `task_info` crate and a `task_basic_info` module within it.

[nnethercote]

The crate provides an interface to the Mac-specific task_info()
function in general, and the module provides an interface to the
TASK_BASIC_INFO flavor. Currently only the virtual_size and
resident_size values of the task_basic_info struct are exposed, but
there's obvious room for expansion.

This is used to implement the -m measurements on Mac.

[highfive]

Warning

• These commits modify unsafe code. Please review it carefully!

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[pcwalton]

Did we decide not to make this a submodule? Anyway, r+

[nnethercote]

Thanks for merging.

Did we decide not to make this a submodule?

In #2720 you suggested an instrumentation module, which I didn't end up doing because "I couldn't work out how to create a new module, and it also felt like overkill." It could certainly be done as a follow-up if necessary.