Explicit backtrace handling

[aantron]

This issue is about looking into storing backtraces explicitly in rejected promises and explicitly extending them during rejection propagation by bind, etc. If this can work, it amounts to a rewrite of Lwt's backtrace handling, and may make the PPX unnecessary for backtraces.

See https://github.com/ocsigen/lwt/issues/714#issuecomment-534992522 for a starting test case.