ivy.el (ivy-resume): automatically resume to last resumable session

[kiennq]

Current when we try to resume to current ivy-last, sometimes it failed with The last session isn't compatible with ivy-resume.
However, we already have a list of resumable sessions stored in ivy--sessions already. That can be used to automatically determine the session to be resumed from when invoked by ivy-resume.

In this change:

• Make ivy--sessions store the session as a stack, so the last used session will always on top
• In case current session is not compatible with ivy-resume, use the top of ivy--sessions as the desired session.

[basil-conto]

Apart from a couple of minor nits, LGTM, thanks.

[kiennq]

@basil-conto Is this okay to be merged?

[basil-conto]

Is this okay to be merged?

It looks fine to me, but that's for @abo-abo to decide, sorry.

[kiennq]

I see, thanks