I've included a compatibility test here to make sure that the numbers
from this are in line with the numbers produced by xHalfs, yThirds, etc.
After this commit I'll introduce a helper function that fills based on a
span specified with this enum to replace any uses of xHalfs and friends.
Once I do that I'll remove them and the compatibility test, this should
be a much cleaner interface for this and make it easier to consistently
align block elements with each other.
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