core: Add ResourceTable

[bartlomieju]

Closes #3134

CC @ry

[bartlomieju]

I tried implementing downcasting manually, but got such errors:

error[E0038]: the trait `resources::Resource` cannot be made into an object
  --> core/resources.rs:61:3
   |
61 |   pub fn add(&mut self, resource: Box<dyn Resource>) -> ResourceId {
   |   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `resources::Resource` cannot be made into an object
   |
   = note: method `downcast_ref` has generic type parameters
   = note: method `downcast_mut` has generic type parameters

That's why I used Downcast trait from downcast-rs crate.

[bartlomieju]

Would be awesome if we could do something like:

let stream = table.get_mut::<dyn AsyncWrite>(rid)?;
stream.poll_write(&zero_copy_buf)

[ry]

Looks great! Thank you for this important work!
I defer to @piscisaureus to approve and land.

[bartlomieju]

Looks great! Thank you for this important work!
I defer to @piscisaureus to approve and land.

Cool, though we shouldn't land it until we figure out how to implement read/write handling

[ry]

Cool, though we shouldn't land it until we figure out how to implement read/write handling

The way you handled it in the example is good.

[bartlomieju]

Cool, though we shouldn't land it until we figure out how to implement read/write handling

The way you handled it in the example is good.

Can't be translated to CLI though, PTAL at this comment

[ry]

Can't be translated to CLI though, PTAL at this comment

I don't understand why it can't work in the CLI. It's the same operation.

[ry]

@piscisaureus approves too - thanks @bartlomieju !