Add a wait-free intrusive stack data structure

[chrisvest]

The WFIStack is added to the set of experimental data structures. This data structure has, from experience, proven useful for implementing a number of other concurrency primitives.

Let me know if this is interesting.

[coveralls]

Pull Request Test Coverage Report for Build 698

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 19 unchanged lines in 4 files lost coverage.
• Overall coverage decreased (-0.2%) to 86.054%

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Files with Coverage Reduction	New Missed Lines	%
jctools-core/src/main/java/org/jctools/maps/NonBlockingHashMapLong.java	2	80.65%
jctools-core/src/main/java/org/jctools/maps/NonBlockingSetInt.java	3	77.65%
jctools-core/src/main/java/org/jctools/maps/NonBlockingHashMap.java	4	79.48%
jctools-core/src/main/java/org/jctools/maps/NonBlockingIdentityHashMap.java	10	75.31%
<!--	Total:	19

Totals
Change from base Build 681:	-0.2%
Covered Lines:	4770
Relevant Lines:	5543

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💛 - Coveralls

[nitsanw]

Thanks @chrisvest ! this is interesting, I'll have a close read next week :-)

[chrisvest]

Currently working on adding a benchmark.