Unwind & backtrace support

[cstanfill]

• Keep track of where eh_frame lives
• Add lots of code to parse the data that lives there
• Call it during panic()
• Just call unwind_from_here() from threads.h if you want to get a backtrace from somewhere
• There's a BIST that checks we can still parse all of eh_frame but it's disabled for now because it's slightly loud at the end.
• Some helpful scripts

Demo: https://gist.github.com/cstanfill/39bc9c81d1e808cea722adeb66ab8b8f This fixes #48

[gsilvis]

Fixes #28