Fault handler

[aykevl]

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When this callback is set, it will be called when a critical error happens (such as a HardFault). Additionally, it will be called when a runtime panic happens if the -panic=trap option is used during compilation.

This PR does not implement handling of regular panics yet.

This PR is marked as WIP as it causes a ~200 byte increase in binary size, which is much higher than I expected. I'll see whether I can reduce that a bit.

@magiconair