replace `next_id` with `alloc_with_id`

[matklad]

fn alloc_with_id(f: impl FnOnce(ID) -> T)

seems like potentially more bullet-proof API?

[fitzgen]

Unless there is also an assertion against re-entrancy, I don't think this actually removes potential bugs resulting from operation orderings like

1. get next id (either via next_id or alloc_with_id)
2. "unexpected" alloc
3. alloc the thing trying to use the id gotten in (1)

I'm not convinced it is worth adding the overhead that an assertion against re-entrancy would require...

[matklad]

alloc takes &mut, so no reentrancy is possible

[fitzgen]

Ah! great point! Yeah, I'd be in favor of this then!