script: Inline some hot functions used in flow constructions.

9% improvement in style recalc on the rainbow page.
Added refs to Element's DOM spec

This is a sub-task for #1660.
Added refs to live HTMLCollections issue
Fixed FIXME message as pointed out by jdm
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
Use DOMString more consistently.
auto merge of #1690 : Ms2ger/servo/DOMStrings, r=jdm
auto merge of #1691 : kmcallister/servo/linked-failure, r=pcwalton

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.
Rename starts_*_flow to is_*_flow
Move DOMString into servo_util.
Add str.rs.
auto merge of #1694 : Ms2ger/servo/move-DOMString, r=SimonSapin
style: Separate out style structs into inherited and initial structs.

This doesn't make much of a difference in performance, but will be
necessary for style struct sharing.