Auto merge of rust-lang#3083 - saethlin:gc-history, r=oli-obk

GC the Stacked Borrows allocation history

This handles the biggest contributor to rust-lang/miri#3080

The benchmark that this adds demonstrates the memory improvement here, but our benchmark setup doesn't record memory usage, and `hyperfine` doesn't support emitting memory usage stats. I ran this benchmark manually with `/usr/bin/time -v cargo +miri miri run` 🤷

Author: bors