Suggestion: attribute form of `submit`

[Diggsey]

Would be used like this:

#[inventory::submit]
static X: Foo = Foo { ... }

This form would be useful when you want to use the value directly as well as "registering" it for use later.

[dtolnay]

I would prefer not to build this into this crate, but an attribute like that could be implemented by a downstream crate.