Adds a Value::world(), analogous to Value::globals(). There may have been some reason for this not to be a thing, but it was in extools without issue, so.
Adds a few ways to get a turf Value: turf_by_id_unchecked, turf_by_id, turf. The first doesn't do bounds checking, so is unsafe; the second does bounds checking on the ID; the third takes coordinates instead of a raw ID, and does its own bounds checking.
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