Stores accept mutable data

[Rich-Harris]

Fixes #2171. TODO implement deep bindings

[codecov-io]

Codecov Report

Merging #2190 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2190   +/-   ##
=======================================
  Coverage   91.83%   91.83%           
=======================================
  Files           1        1           
  Lines          49       49           
=======================================
  Hits           45       45           
  Misses          4        4

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 06040d3...093cc0f. Read the comment docs.

[Rich-Harris]

Spotted an opportunity to shrink store.mjs while I was at it — readable is now implemented in terms of writable