Support cycle/previous-focus

Define a `cycle/previous-focus` function that returns the item that was
previously "focused" in the cycle. This is helpful for toggling back-and-forth
between buffers and EXWM workspaces for example without needing to define ad-hoc
variables to support it.

Also: Adds tests to cycle.el.

Also: Prefers `struct/set!` instead of `setf`. See the previous commit's message
for more information about that preference.

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