Improve the inner confusing address naming

[XuyangSong]

As described in #108 :

The user_address(user_address = Com( Com(send_vp, nk), recv_vp)) will be removed. Instead, add an inner address variable(address = hash(vp_data_nonhashed, nk_com)). And the address will go to NoteCommitment. Does it make sense that we still use an address here? Otherwise, the vp_data_nonhashed and nk_com will go to NoteCommitment independently, and we need to extend the SinsemillaCommit since we add another 255 bits.

Rename the inner address or extend the SinsemillaCommit.

[vveiln]

I would say that extending the note commitment would make more sense if we don't need to use this variable somewhere else. I looked through the code quickly and it seems like there is no special use for that structure, but I might be wrong.

If there is a special use (that doesn't apply to other fields of the note, e.g. some special check for app_data_dynamic and nk_com), renaming would work better, but it is hard to tell what should be the new name then

[XuyangSong]

Fix it after we have the key design. #132