Build libstd etc. with debug info when Servo configured with --enable-debug
Change the mechanism behind summit-crash.html
Move FailureMsg contents into a cloneable struct
Add a helper for sending a message on task failure
Restore failure handling

We probably leak some threads and resources, e.g. when the script task crashes
and doesn't get a chance to send layout data back to layout to be deallocated.

Not tested with iframes yet.
Restore hard-fail mode
Use hard-fail for reftests
Find failure.html relative to exe path, not cwd
Move DOMString into servo_util.
Add str.rs.
Initial work on UCS-2 DOMStrings.
Save progress.
Fix HTMLImageElement.
Fix more build errors.
Pass a slice to xml_name_type.
Push to a mutable string.
Fix borrowck errors and warnings in script.
Start on layout.
Fix some build errors.
More build fixes.