small update to implementation of mutation storing

[petrelharp]

I've updated this to include a few more details, I think reflecting the current state of things?

In writing this up I wondered about saving/adding mutations, which led to #646, so there might be another note to add.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.72%. Comparing base (07fc977) to head (e09adce).

Additional details and impacted files

@@              Coverage Diff               @@
##           multitrait     #647      +/-   ##
==============================================
- Coverage       76.81%   76.72%   -0.09%     
==============================================
  Files             118      118              
  Lines           78599    78599              
  Branches        14240    14241       +1     
==============================================
- Hits            60372    60302      -70     
- Misses          18227    18297      +70     

see 11 files with indirect coverage changes

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[bhaller]

It's hard to read the diffs on this kind of thing, so I've just assumed that you have done Good Things, and merged. I'll do another pass over it when #648 has been resolved in one way or another. :->