[scudo] Add a Timer class to assist performance measurement
Add Timer and TimingManager which provide convenient way to meause the
execution time of code snippets. The output looks like,
```
-- Average Operation Time -- -- Name (# of Calls) --
1747.2(ns) popBatch (59)
92.3(ns) popBatchImpl (73)
101.6(ns) EmptyBatchProcess (5)
2587.0(ns) pushBlocksImpl (13)
```
Note that `EmptyBatchProcess` is nested under the timer `popBatchImpl`.
Reviewed By: cferris
Differential Revision: https://reviews.llvm.org/D143626