DDIR server: imports arrive as the producer's arrangement

[frankmcsherry]

A program that imported a published trace received it as rows (or, in columnar mode, as containers), and each of its joins, reduces, and arranges of the import built a private copy of an arrangement the producer already maintains. Imports now arrive as that arrangement, entered into the program's scope (TraceEnter), on both backends.

vec: Arr is a local or an imported arrangement. Join and reduce take either through the cursor tactics, as an entered batch's cursor lifts its times.

corgi: times are integer lanes, and a host epoch e and the program time (e, []) have the same lanes, so an imported chunk is read at the program's time by sharing it: CorgiChunk::retime casts the Rc, which repr(C) on the chunk's contents and ColTimes makes sound. RetimeJoin and RetimeReduce wrap the proxy tactics and retime imported batches as they cross in, one Rc per chunk. Columnar exports, generated sources included, reach columnar programs this way.

An imported arrangement also enters child scopes, iterative ones included, as a program's own arrangements do (#931). An import that a scope also reads as rows is flattened once per scope, and the rows enter its children with the arrangement.

Server::set_arranged_imports(false), or DDIR_ARRANGED_IMPORTS=0 for the server binary, restores rows (containers in columnar mode). A shape ascription on an arranged import is checked by a side operator over the trace's batches, which produces nothing: on vec always, on corgi when the export's shape was not known at install (as #927 checks containers).

examples/import_bench: one producer of 1M edges, then 8 consumers each joining the import twice, 1000 edges replaced per tick. Corgi without arranged imports reads containers (#927).

                             vec              corgi
                          rows   arranged   containers   arranged
install-to-answer, ms     ~400   ~3         27-42        ~1
memory per consumer, MiB  ~95    ~0.5       ~54          ~0.3
tick, 8 consumers, ms     13.3   2.0        3.1          0.9

Consumers that join the import inside a loop (reachability) are alike. After 500 ticks of churn the published trace holds the same updates with 0 or 8 importers sharing it.

Tests: answers with and without arranged imports agree on each backend, and the backends agree, for a join and a reduce of an import at the root, a join inside a loop, and the import read as rows, through changes and an importer installed late. A declared import of an export of unknown shape fails on mismatched data when read in place.