Move HAMT into filecoin-project and take over ownership

[anorth]

The HAMT implementation in https://github.com/ipfs/go-hamt-ipld/pull/52 is specialized for, and used only by, Filecoin. And within Filecoin, it's used primarily by this repo: the only other use I'm aware of is as the root of the state tree (which perhaps should be in this repo too #117)

@rvagg and @Stebalien have proposed some solid improvements, which we should shepherd in.

Moving the repo will cause a Go package path change which downstream repos will need to integrate.

[anorth]

See https://github.com/ipfs/go-hamt-ipld/issues/51