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ARROW-67: C++ metadata flatbuffer serialization and data movement to memory maps

Several things here:

* Add Google flatbuffers dependency
* Flatbuffers IDL draft in collaboration with @jacques-n and @stevenmphillips
* Add Schema wrapper in Cython
* arrow::Schema conversion to/from flatbuffer representation
* Remove unneeded physical layout types from type.h
* Refactor ListType to be a nested type with a single child
* Implement shared memory round-trip for numeric row batches
* mmap-based shared memory interface and MemorySource abstract API

Quite a bit of judicious code cleaning and consolidation as part of this. For example, List types are now internally equivalent to a nested type with 1 named child field (versus a struct, which can have any number of child fields).

Associated JIRAs: ARROW-48, ARROW-57, ARROW-58

Author: Wes McKinney <wesm@apache.org>

Closes #28 from wesm/cpp-ipc-draft and squashes the following commits:

0cef7ea [Wes McKinney] Add NullArray type now that Array is virtual, fix pyarrow build
5e841f7 [Wes McKinney] Create explicit PrimitiveArray subclasses to avoid unwanted template instantiation
6fa6319 [Wes McKinney] ARROW-28: Draft C++ shared memory IPC workflow and related refactoring / scaffolding / cleaning.