Let's adapt this class a bit better to how it's actually being used.
Instead of having valid/invalid states and storing an error in case
it's invalid, a MappedFile is now always valid, and the factory
function that creates it will return an OSError if mapping fails.
Creating a backtrace from a crashdump already existed as a few
standalone functions in CrashDaemon. This patch refactors the code
required for this to make it generally usable and moves it to
CoreDump::Backtrace, which provides both raw data as well as
stringification.