Use term StakeShuffle

[xaur]

I suggest changing all occurrences of CoinShuffle++ to StakeShuffle.

The release page and possibly other places use term CoinShuffle++ to refer to our mixing design, but it is incorrect (CSPP is only one part of the system) and possibly not too catchy PR-wise. Luckily, someone has come up with term StakeShuffle and some contractors seem to like it. It was also used in the January press release.

See decred/dcrdocs#1152 for more details.

[xaur]

Update - per the discussion in decred/dcrdocs#1153 it seems we need to discuss this further before continuing with the "rebrand", let's put this issue on hold for now.

[xaur]

#982 updated the release page to use StakeShuffle (waiting for deployment as of writing).