Before this commit, `at()` and `operator[]` did the same thing, making
one of them redundant. There was also a somewhat awkward `parts_at()`,
which is more generic than `at()`.
After this commit, `parts_at()` becomes `at()`, and `at()` becomes
`operator[]`. Any quick-and-dirty accesses should use `items["name"]`,
which expects a single-part item. Generic code should instead use
`items.at("name")`, which will return multiple parts. Both will check
for the existence of the name.
This is unlikely to break downstream code since it's likely been using
the shorter `operator[]`. (In any case this API is not stable.)
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