Create prettier messages when fcd crashes

This commit prints a program stack trace and a 'logical stack trace' (built from annotation objects sprinkled in important source locations) on abnormal termination.

Fix crash in AstContext when trying to create intrinsics without a mo…

…dule

Use special function to access pointers in fs and gs segments

Last commit known to work with LLVM 3.7.

This could be considered "stable release 0" (with lots of quotes around "stable").