Introduce SkArenaAlloc - should be fast for POD types and RAII for types with dtors.

- Implementation.
- Use in SkLinearPipeline.
 
TBR=mtklein@google.com
Change-Id: Ia8efd09b2f3139a57182889ba84d1610eae92749
Reviewed-on: https://skia-review.googlesource.com/6352
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Herb Derby <herb@google.com>
8 files changed