Optimized constant SignalProducers.

[andersio]

These speedups are applicable to only closure-based observation APIs (i.e. startWithSignal is ineligible).

Item	testSequence (32 items)	testValue
master	avg 30296 ns; min 26426 ns	avg 8910 ns; min 8171 ns
unsafeAction	avg 16092 ns; min 14627 ns	avg 2928 ns; min 2568 ns
+unsafeUninterruptibleAction	avg 16140 ns; min 14502 ns	avg 664 ns; min 587 ns
Speedup	~1.85x	~13.5x

Benchmark source.

This PR also relaxed UnsafeAtomicState.is.

Checklist

• Updated CHANGELOG.md.

[andersio]

This is not ready though. It needs way more changes so that not only direct observation to constant SignalProducers is benefited. 😕

[andersio]

Some interesting numbers:

@testSequence(): avg 15598 ns; min 14388 ns
@testFlatMapSequence(): avg 152921 ns; min 137964 ns

@testValue(): avg 660 ns; min 591 ns
@testFlatMapValue(): avg 54609 ns; min 49320 ns

testFlatMapSequence is basically testSequence but in the form of [2, 4, 8, 16].flatMap { repeat 1 for $0 times }.

:/