libstage: deprecate stage_node_*, go heirarchical

Now stage_t is the only type, and a tree of stages may be
constructed by supplying a parent to stage_new().

Essentially stage_t swallowed stage_node_t.

The heirarchy is handy for grouping stages logically, and alpha
is computed statefully during the recursive render.  So alpha of
grouped stages may be varied at a single parent.  The active
flag is also honored recursively for a similar effect; toggle
subtrees in a single place.

Container stages may be created by omitting object/render/free
attributes.  These are handy for the aforementioned group
alpha/active controls.

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