feat(core): add distinctUntilChanged operator

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[raveclassic]

Hey @tamazlykar, what is the reasoning behind adding this operator? Property's are designed so that value comparison is an implementation detail and I wouldn't want to expose any kinds of "comparators" to the end user. By design, values are always compared by reference, if you need deep comparison this usually means the structure is mutated. Any kinds of mutations and similar impurities are out of scope of frp-ts

[raveclassic]

Ok, I see you need deep comparison due to different object literals (different references) even their contents are the same. Well, I can't say such things are in the spirit of frp-ts as the intention is clearly to set the new reference and deep comparison looks like a "fix" for that. Instead I'd suggest moving the comparison out of the set to a condition wrapping that set.

const a = newAtom({ name: 'foo' })
a.set({ name: 'foo' }) // the intention is to push new object

if (!deepEqual(a.get(), {name: 'foo'})) { // the intention is to check deeply
  a.set({ name: 'foo' }) 
}

[raveclassic]

ok found the original issue, linking https://github.com/raveclassic/frp-ts/issues/59