Adding store save

[MLewiDev]

Added .save function to stores.
It uses logic from single model and applies it to the whole store.
Tests added based on the model tests.

[daanvdk]

So one thing that we should look into if we want to add it:

This currently does not handle saving deletions from the top level store, this is supported on the binder API level in the multiput (next to data you can also send deletions with a list of ids that have to be deleted.).

However this means we should start to keep track on the store which ids have been removed from it. (Or which ids it originally had and on save just check which ones are not in there anymore)

[codecov-commenter]

Codecov Report

Merging #134 (eeb7cab) into master (cec7709) will increase coverage by 0.12%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #134      +/-   ##
==========================================
+ Coverage   93.63%   93.75%   +0.12%     
==========================================
  Files           5        5              
  Lines         848      865      +17     
  Branches      201      204       +3     
==========================================
+ Hits          794      811      +17     
  Misses         44       44              
  Partials       10       10              

Impacted Files	Coverage Δ
src/Model.js	91.01% <100.00%> (+0.04%)	⬆️
src/Store.js	100.00% <100.00%> (ø)

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[BlayeeR]

It should also call this.clearSetChanges after successful save, otherwise this.hasUserChanges will always stay true even after saving the store. I encountered this issue in a project that already uses changes from this PR