coverage: Simplify how counter terms are stored

Using `SmallVec` here was fine when it was a module-private detail, but if we
want to pass these terms across query boundaries then it's not worth the extra
hassle.

Replacing a method call with direct field access is slightly simpler.

Using the name `counter_terms` is more consistent with other code that tries to
maintain a distinction between (physical) "counters" and "expressions".

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