std provides a panic handler; without it we must provide our own.
It can also be provided by another crate, such as panic-halt.
Depending on the target, you may need to compile with panic = "abort" to avoid an error about
Note that there is no main or any other entry point; it’s up to you to define your own entry
point. This will typically involve a linker script and some assembly code to set things up ready
for Rust code to run.