Print backtrace upon panic

[jounathaen]

This is not an easy one, but it would make debugging hermit way more easy, if we had a RUST_BACKTRACE=1 like backtrace in the panic message.
We would probably need to analyze the stack manually for this and somehow map back the function names from the debug info to the frame pointers.

Could help e.g., in cases like:

• cargo test --test fs-test fails on main uhyve#726