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

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.
Merge pull request #2786 from nnethercote/task_info

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