Erased almost every instance of WORLD_SIZE and replaced it with a local
power of two, map_size_lg (which respects certain invariants; see
common/src/terrain/map.rs for more details about MapSizeLg). This also
means we can avoid a dependency on the world crate from client, as
desired.
Now that the rest of the code is not expecting a fixed WORLD_SIZE, the
next step is to arrange for maps to store their world size, and to use
that world size as a basis prior to loading the map (as well, probably,
as prior to configuring some of the noise functions).
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