Auto merge of #1559 - Aaron1011:new-miri-backtrace, r=RalfJung

Add API for capturing backtrace

This PR adds two new Miri-defined extern functions:
`miri_get_backtrace` and `miri_resolve_frame`, which are documented in
the README. Together, they allow obtaining a backtrace for the currently
executing program.

I've added a test showing how these APIs are used. I've also prepared a
companion PR `backtrace-rs`, which will allow
`backtrace::Backtrace::new()` to work automatically under Miri.

Once these two PRs are merged, we will be able to print backtraces from
the normal Rust panic hook (since libstd is now using backtrace-rs).

A few notes:
* Resolving the backtrace frames is *very* slow - you can actually see
  each line being printed out one at a time. Some local testing showed
  that this is not (primrary) caused by resolving a `Span` - it seems
  to be just Miri being slow.
* For the first time, we now interact directly with a user-defined
  struct (instead of just executing the user-provided MIR that
  manipulates the struct). To allow for future changes, I've added
  a 'version' parameter (currently required to be 0). This should allow
  us to change the `MiriFrame` struct should the need ever arise.
* I used the approach suggested by `@oli-obk` - a returned backtrace
  pointer consists of a base function allocation, with the 'offset'
  used to encode the `Span.lo`. This allows losslessly reconstructing
  the location information in `miri_resolve_frame`.
* There are a few quirks on the `backtrace-rs` side:
  * `backtrace-rs` calls `getcwd()` by default to try to simplify
    the filename. This results in an isolation error by default,
    which could be annoying when printing a backtrace from libstd.
  * `backtrace-rs` tries to remove 'internal' frames (everything between
     the call to `Backtrace::new()` and the internal API call made by
     backtrace-rs) by comparing the returned frame pointer value to
     a Rust function pointer. This doesn't work due to the way we
     construct the frame pointers passed to the caller. We could
     attempt to support this kind of comparison, or just add a
    `#[cfg(miri)]` and ignore the frames ourselves.