Disposal is not called with `use!` and the like

[abelbraaksma]

It appears that Dispose() or DisposeAsync() is not being called when use! is used. First mentioned offline by @bartelink. Logging here to link the PR to a tracking issue and for the changelog/release notes.

Turned out that this was inadvertently commented out in 27486f3, which shouldn't have happened. In a follow up I will create some tests to ensure this won't happen again.

[abelbraaksma]

This will be fixed in #149 and deployed as an interim release as 0.4.0 alpha. I'll release tomorrow (#159).