workspace_squash is container_flatten in the reverse
direction. Instead of eliminating redundant splits that are
parents of the target container, it eliminates pairs of
redundant H/V splits that are children of the workspace.
Splits are redundant if a con and its grandchild have the
same layout, and the immediate child has the opposite split.
For example, layouts are transformed like:
H[V[H[app1 app2]] app3] -> H[app1 app2 app3]
i3 uses this operation to simplify the tree after moving
heavily nested containers to a higher level in the tree via
an orthogonal move.
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