Deprecate minusAssign/plusAssign operators from public API

[mvicsokolova]

I suggest to remove minusAssign/plusAssign operators from public API of AtomicInt and AtomicLong.

• these operators are not consistent with the API of Java atomics, K/N atomics and it's better to use addAndGet instead.
• this API is not widely used (though I should check that properly)
• these operators were added just because they seemed useful

Not the reasons to remove the API, but related problems:

• this API is not supported by the compiler plugin and causes problems like this: IllegalStateException: No minusAssign function found in minusAssign for IR transformation #274
• also it is not covered by tests

Alternative solution is to solve the problems above and keep the API.
And of course it's more convenient to write:

private val x = atomic(0)

x -= 5

vs

private val x = atomic(0)

x.addAndGet(-5)

@fzhinkin WDYT? 👀

[lppedd]

IMO the operator functions feel more natural. And I can swap an atomic with a normal number type (and vice-versa) without changes.

People working with Kotlin should be aware of operator functions, and the IDE provides navigation to the target in case one wants to explore the code.

[mvicsokolova]

That's a good point, right.

[mvicsokolova]

These operators will be supported in the atomicfu compiler plugin for the future Kotlin versions: KT-67839