Keep app states in the tree store

When the data host keeps states (PureData.state and .save, implemented by
pure_tree_data), every state of an app lives in the store: a text state's
draft is its stored value, a list mirrors each change into a stored draft
(record header, then items), and numbers, cursors and indices are stored
together by name. An app starts from what is stored, moves list fields by
name when a record changed, resets cursors and indices that no longer fit,
and refuses damaged stored values.

Each key, text and message event publishes the states it changed together
with the message's document in one store commit, or fails and leaves
memory and store as they were. Frame changes go with the next event or
pure_app_save. pure-run takes --store PATH.

Tests run the same random events on an app in memory and on a stored app
that restarts every few events and require equal states after every step;
kill a process at every commit stage of an event that changes a number, a
text and a list; and check frames, message atomicity and migration by
restart and by replacement. A stored event costs about 10.6 ms more on the
Pi's SD card, mostly two fsyncs.

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