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

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

- Implementation.
- Use in SkLinearPipeline.

TBR=mtklein@google.com

Change-Id: Ie014184469b217132b0307b5a9ae40c0c60e5fc9
Reviewed-on: https://skia-review.googlesource.com/6921
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Herb Derby <herb@google.com>