Remove any .unwrap() and panic!s

[Jengamon]

For example:

ruwren/src/runtime.rs

Line 97 in eb68c25

ffi::CString::new(string).unwrap_or_else(|_| panic!("Failed to convert source to C string for {}", module_name.to_string_lossy())).into_raw()

unwrapping or panicking in runtime.rs is UB as it crosses the C threshold. Remove these and replace with some kind of defaulting.