Cleanly handle runtime issues

[jaseemabid]

Panics in the runtime code prints this to stderr and gets segfaulted.

 ✔  …/inc/295c5a3b  LD_LIBRARY_PATH=~/src/inc/rs/target/debug ./inc
thread '<unnamed>' panicked at 'assertion failed: (val & MASK) == STR', src/rt.rs:131:5
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
fatal runtime error: failed to initiate panic, error 5

The test output is pretty crap in this case making debugging way harder.

[jaseemabid]

Fixed in 35df3e9