Get some stuff out of the common module, which is growing large.
Also migrate the tests into "native" Rust tests so they will run in parallel.
We have to use an explicit setlocale() call to get a multibyte locale, for the
"crazy" tests.
This shows some of the ugliness of the rust borrow checker when it comes to
safely implementing any sort of recursive access and the need to be overly
explicit about which types are actually used across threads and which aren't.
We're forced to use an `Arc` for `ItemMaker` (née `item_maker_t`) because
there's no other way to make it clear that its lifetime will last longer than
the FdMonitor's. But once we've created an `Arc<T>` we can't call
`Arc::get_mut()` to get an `&mut T` once we've created even a single weak
reference to the Arc (because that weak ref could be upgraded to a strong ref at
any time). This means we need to finish configuring any non-atomic properties
(such as `ItemMaker::always_exit`) before we initialize the callback (which
needs an `Arc<ItemMaker>` to do its thing).
Because rust doesn't like self-referential types and because of the fact that we
now need to create both the `ItemMaker` and the `FdMonitorItem` separately
before we set the callback (at which point it becomes impossible to get a
mutable reference to the `ItemMaker`), `ItemMaker::item` is dropped from the
struct and we instead have the "constructor" for `ItemMaker` take a reference to
an `FdMonitor` instance and directly add itself to the monitor's set, meaning we
don't need to move the item out of the `ItemMaker` in order to add it to the
`FdMonitor` set later.
