Explain SIGSEGV backtrace handler

...to both compiler contributors and other rustc invokers.
The previous error messaging was extremely unfriendly,
and potentially both confusing and alarming.

The entire modules is extracted into a new file to help
ease of reading, and plenty of comments get layered in.
The design of the messaging is focused on preventing
the overall purpose of the output from being too opaque
in common cases, so users understand why it is there.
There's not an immediate need for it being actionable,
but some suggestions are offered anyways.

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