create internal and private packages

[schomatis]

First attempt at using the internal package to reduce unnecessary API exposure. After this is merged we can evaluate moving other artifacts from the shard testing if necessary.

[whyrusleeping]

dont use internal, private or similar works better and doesnt punish people for no reason.

[aschmahmann]

@whyrusleeping this is exclusively used for testing. However, it seems fine to move a utility creating a full HAMT into a package called private and leave people to their own demise.

However, #99 has some changes that I would like to move into the internal package such as a flag for being able to change the hash function used by the HAMT shards (e.g. to the identity function for testing). I do not want people to think that that's something they can do without it being really inconvenient for them. At the moment UnixFS only supports are particular brand of murmur3 and nothing else.

@schomatis could you move the CreateCompleteHAMT into a private package, and then move all of the identity hash pieces into an internal package?

[schomatis]

@aschmahmann Refactored into both packages. Can you take another look, please?