Bound what an app holds (--memory): refuse what would grow past it

pure-serve --memory BYTES gives the app a budget of state bytes: its
lists' items as they are stored, its texts, the outbox and the numbers
heard (docs/design/013-memory-budget.md). The count is deterministic,
the same on any device. An event after which the states hold more than
the budget, and more than before it, fails with the fault memory and
changes nothing; HTTP answers 503 memory. One that frees or keeps them
goes on, so an app over its budget can always shrink.

Sends waiting for their receivers are state: a full outbox refuses the
next send until some are stored, which is the backpressure a slow
receiver puts on clients. A delivery refused for memory is not heard:
the link keeps it (c9c0edf) and HTTP senders try again after 503.

An answer (--reply) now takes its send out of the outbox in its own
event (pure_app_answer), so it has the room its send held. Apart, an app
whose outbox filled the budget could never apply an answer that keeps
anything, and the send leaves only after its answer: both would stay for
good. An answer that still does not fit waits, with the wire's after it,
and is tried again every 100 ms: pure_wire_answer leaves it in place
until pure_wire_answered.

Checks: test-module memory_budget (refused and undone, freeing over the
budget, a full outbox, a delivery not heard, answers that fit or wait,
across a restart); check.py server_memory (a sender refused 503 memory
with its receiver down and taking buys once it is up; a receiver
restarted below what it keeps, refusing 7 times in 2 s, then taking all)
and server_answers_memory (answers wait while receipts fill the budget,
then apply once each; the API called once per order). The stats report
state_bytes and memory_refused.

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