Added some explanatory text to the Compact Backtrace Format
documentation, and also to the `CMakeLists.txt` for the runtime.
Tweaked the conversions for `Backtrace.Address` to truncate, which
should result in reasonable behaviour for negative fixed-width
integers.
Use a constant for the ASCII values for `/` and `\` in the Compact
Image Format encoder/decoder.
Make the TSD key for `ElfImageCache` non-optional, and call
`fatalError()` if we don't get one.
rdar://124913332
Move the backtracing code into a new Runtime module. This means renaming
the Swift Runtime's CMake target because otherwise there will be a name
clash.
rdar://124913332
