RUST_BACKTRACE=1 occasionally causes memory explosion

[jdm]

We've seen OOM errors occur on Servo's CI infrastructure before, and narrowed them down to the presence of RUST_BACKTRACE=1 and Servo panicking. A contributor did some more digging and came up with this analysis: https://groups.google.com/forum/#!topic/mozilla.dev.servo/UyR2fjiKahk

[steveklabnik]

Triage: #24344 was merged, so closing