Remove `CowBoxSymStr`.

`CowBoxSymStr` is a type that either holds a `SymbolStr` (which is much
the same as a `Symbol`), or an owned string. When computing skeletons,
a `SymbolStr` is stored if the skeleton is the same as the original
string, otherwise an owned string is stored.

So, basically, `CowBoxSymStr` is a type for string interning. But we
already have one of those: `Symbol` itself. This PR removes
`CowBoxSymStr`, using `Symbol` instead. A good thing about this is that
it avoids storing `SymbolStr` values in `skeleton_map`, something that
is discouraged.

The PR also inlines and removes the `calc_skeleton()` function because
that simplifies the code.

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