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