Yank out the "tier" type and just use operations instead.

Although the concept of a "tier" is useful (to distinguish changes
in the tier from the things being changed), the actual data type isn't
needed.  We can just store an operation that computes the tier of a
step given a minimal tier.

Throwing out tier objects yields several benefits.  It makes the code
somewhat simpler by reducing the number of complex object types
floating around.  It will make it easier to clean up the nomenclature
by talking about costs instead of tiers.  And it will make it easier
to impose a "cost ceiling", just by virtue of making the cost-tracking
subsystem simpler.

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