Having multiple kinds of node that hold numeric values made things more
complicated than they needed to be, and we were already converting
ConstantCalculationNodes to NumericCalculationNodes in the first
simplification pass that happens at parse-time, so they didn't exist
after that.
As noted, the spec allows for other contexts to introduce their own
numeric keywords, which might be resolved later than parse-time. We'll
need a different mechanism to support those, but
ConstantCalculationNode could not have done so anyway.
Before this change, we only parsed fit-content as a standalone keyword,
but CSS-SIZING-3 added it as a function as well. I don't know of
anything else in CSS that is overloaded like this, so it ends up looking
a little awkward in the implementation.
Note that a lot of code had already been prepped for fit-content values
to have an argument, we just weren't parsing it.
At computed-value time, this is converted to whatever the parent's
computed value is. So it behaves a little like `inherit`, except that
an inherited start/end value uses the parent's start/end, which might
be different from the child's.
