Add timed variant for IO[Unit]

[geirolz]

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Hello folks! I'm not sure about the solution but I encountered this "problem" enough times to submit this PR. Basically when you want to use timed on an IO[Unit] you have to map the result to just keep the left side of the tuple. It would be nice to have a method to this directly within IO.

My proposal here introduces breaking changes, other more conservative solutions would be to use a dedicated name for this case such as:

• timed_
• timedIgnore (with or without and )
• timedL or timedR it depends on which side you see it
• timedOnly

Let me know what do you think! Thank :)