This crate re-exports the `Backtrace` type at top-level from a nested
module. `Backtrace` in turn has `Frame` in its public API, which is not
re-exported anywhere. This is legal and external users can call methods
on `Frame`, but it doesn't appear in the rustdocs, making it
unnecessarily difficult to figure out how to use this API. Re-exporting
`Frame` fixes that, and also allows naming the type directly if needed.
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