Summary:
- AssertionError when initialized with Non-Direct Buffer
- Tests + coverage for DirectSlice
- Slice sigsegv fixes when initializing from String and byte arrays
- Slice Tests
Test Plan: Run tests without source modifications.
Reviewers: yhchiang, adamretter, ankgup87
Subscribers: dhruba
Differential Revision: https://reviews.facebook.net/D30081
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