Better stack traces

[RalfJung]

Stack traces can be pretty unreadable currently, because we are showing the code of all call sites, including stuff somewhere deep in libstd that's between the startn-fn lang item and main.

We should be smarter and not print code snippets for all call sites.

[RalfJung]

Backtraces are better now since we don't show code outside the current crate.

But I still find one thing pretty confusing: we print things like

   = note: inside call to `std::alloc::alloc` at /home/r/.rustup/toolchains/miri/lib/rustlib/src/rust/src/liballoc/alloc.rs:172:35
   = note: inside call to `<std::alloc::Global as std::alloc::AllocRef>::alloc` at /home/r/.rustup/toolchains/miri/lib/rustlib/src/rust/src/liballoc/alloc.rs:225:15
   = note: inside call to `alloc::alloc::exchange_malloc` at /home/r/.rustup/toolchains/miri/lib/rustlib/src/rust/src/liballoc/rc.rs:327:49

Usually, backtraces pair up a function name with the relevant file and line inside that function. But Miri pairs up a function name with the file and line of its caller. I think that is unnecessarily confusing.