Add Rust support to gdb

[brson]

Calling functions in gdb among other things requires direct debugger support. We'll have to start modifying gdb.

[steveklabnik]

I'm pulling a massive triage effort to get us ready for 1.0. As part of this, I'm moving stuff that's wishlist-like to the RFCs repo, as that's where major new things should get discussed/prioritized.

This issue has been moved to the RFCs repo: rust-lang/rfcs#767