PieceCID is confusing term and is not a CID, should be renamed PieceCommitment

PieceCID is repeatedly confused with PayloadCID, and it often takes long explanations to help people understand it is not the root of the IPLD Graph of the data you want to store, but instead the value we call "CommP" -- which stands for PieceCommitment.

Moreover, the value is not infact a CID -- it lacks multibase information, it does not conform to any CID spec, and moreover is not itself ultimately a hash of bytes!

I propose, and I understand this might make for a sizable find/replace, that PieceCID be renamed PieceCommitment, to clearly identify that it is not in fact a CID, and definitely not the root of the IPLD graph of data a client is intending to store.

I believe the time spent to changing names will easily be make up for time lost gaining understanding on what this term means.