feat(option): add Option::clear method

[cathaysia]

Add Option::clear. This makes Option more like a container.

Although this can also be achieved by using let _ = Option::take(), considering that Option provides insert method (somewhat similar to a container), it seems reasonable to add a clear method to it.

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[scottmcm]

If you'd like to add a new method, please file an ACP to get libs-api's initial feedback on whether it should exist.

Note that _ = x.take(); is a pretty weird way to phrase this -- I'd expect just x = None;.