Combining libarrow/libarrow_io/libarrow_ipc

[wesm]

See apache/arrow#516 -- the idea is to make building / linking simpler for users who are only using the core libraries. Since in many cases you would use all of these libraries, I thought it made sense to make them a single library

Figured I would do all the disruptive toolchain changes at once; I don't expect to make it a regular occurrence

[robertnishihara]

Addressed by #464.