Bump backtrace-rs

Aaron1011 · Aaron1011 · commit 865c30d52ac1 · 2020-10-15T15:21:12.000-04:00
This pulls in rust-lang/backtrace-rs#376, which fixes Miri support for `std::backtrace::Backtrace`.
diff --git a/library/backtrace b/library/backtrace
@@ -1 +1 @@
-Subproject commit 893fbb23688e98376e54c26b59432a2966a8cc96
+Subproject commit a6dd47bd588c882e735675a1379d2b61719fa380
