prevent cyclical store computations, and computation duplication

[Rich-Harris]

Something @adamhaile spotted yesterday (thanks for the eagle eye!) — when Store re-sorts its computed properties, it doesn't bother to mark which ones it's already seen, so in a situation like this...

store.compute('b', ['a'], a => a * 2);
store.compute('c', ['b'], b => b * 3);
store.compute('d', ['b'], b => b * 4);

...where both c and d depend on a computed value, it would end up thinking it had as many as 5 things to recompute on each state change, rather than a maximum of 3.

I also added some cyclical dependency detection, so that this...

store.compute('a', ['b'], b => b + 1);
store.compute('b', ['a'], a => a + 1);

...results in a helpful 'Cyclical dependency detected — a computed property cannot indirectly depend on itself' error, rather than a stack overflow.

[Conduitry]

Probably shouldn't use template literals in code that's going to get shipped to users.

It's also bugging me a bit having this whole error message text end up in production code, but I don't see a reasonable way around that.

[Rich-Harris]

Oops, good catch. Should perhaps add a test for ES6+isms. I agree about the error message. Maybe we could just shorten it? e.g. just 'Cyclical dependency detected'

[Conduitry]

👍 Using a shorter error message seems like a reasonable compromise.

[codecov-io]

Codecov Report

Merging #964 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #964   +/-   ##
=======================================
  Coverage   91.76%   91.76%           
=======================================
  Files         109      109           
  Lines        4054     4054           
  Branches     1301     1301           
=======================================
  Hits         3720     3720           
  Misses        150      150           
  Partials      184      184

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