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ARROW-750: [Format] [C++] Add LargeBinary and LargeString types

These are like Binary and String respectively, except with 64-bit offsets
so as to allow extremely large individual values.

Closes #4921 from pitrou/ARROW-750-large-binary-2 and squashes the following commits:

3e17dcd27 <Antoine Pitrou> Address review comments
9a2995378 <Antoine Pitrou> ARROW-750:   Add LargeBinary and LargeString types

Authored-by: Antoine Pitrou <antoine@python.org>
Signed-off-by: Antoine Pitrou <antoine@python.org>