minor refactor of profiler
add tests for profiler buckets
add reference to issue regarding CTFE function on enum types for length
use HashMap instead of vector for profiler buckets
use TreeMap instead of HashMap for profiler buckets
reorder some struct declarations in profiler
auto merge of #808 : tikue/servo/master, r=metajack

* Use ProfilerBucket structs instead of (ProfilerCategory, ~[Float]) tuples
* Simplify the order check of bucket categories