Move go-ipfs/bitswap package to go-ipfs/exchange/bitswap
* Delineates the difference between the generic exchange interface and
implementations (eg. BitSwap protocol)
Thus, the bitswap protocol can be refined without having to overthink
how future exchanges will work. Aspects common to BitSwap and other
exchanges can be extracted out to the exchange package in piecemeal.
Future exchange implementations can be placed in sibling packages next
to exchange/bitswap. (eg. exchange/multilateral)
