[RFA] Handle dereferencing Rust trait objects

[Tom Tromey]

>>>>> "Tom" == Tom Tromey <tom@tromey.com> writes:

Tom> Objects that are referenced via a trait can't currently be inspected by
Tom> gdb.  This patch implements the Rust equivalent of "set print object".

I forgot a couple of details.

The Rust compiler patch is here:

    https://github.com/rust-lang/rust/pull/45897

It should go in soon.

I wasn't sure if I ought to xfail the new tests.  I have the impression
that I'm the only person who runs the Rust gdb tests, but I'm not
actually certain.

Tom