Allow users access to Backtraces fields
## Motivation
Exception reporting frameworks (eg. Sentry) accept stacktrace information via their APIs.
It's currently not possible to provide this information for Haskell exceptions, because
the only way to access Backtraces is through `displayBacktraces`. It would be convenient
to have some kind of public data type representing backtrace information so that it could
be serialised and used with these frameworks
## Proposal
Expose the Backtraces constructor/fields (caveat emptor). Or, if the internal representation is
likely to change, provide a public interface for accessing execution stack locations/IPE table stack entries/etc
issue