Expose popcount

[polux]

Most CPUs have a popcount (http://en.wikipedia.org/wiki/Hamming_weight) instruction but it is not accessible in Dart.

Yet certain algorithms and datastructures rely on it (http://en.wikipedia.org/wiki/Hash_array_mapped_trie for instance).

So far I have used software emulations of popcount but it's a pain because
  1. it's potentially slower than a C++ version and definitely slower than a CPU instruction
  2. I don't know any fast implementation that works on both 32 and 64 bits architectures

Would it be possible to expose it as a method of int or num? (With a fallback to a C++ version in the VM for CPUs that don't support it and a software version in dart2js.)

[rakudrama]

See also Issue #5798, Issue #6486.

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Added Area-Library, Library-Core, Triaged labels.

[floitschG]

Added this to the Later milestone.
Added Duplicate label.
Marked as being merged into #5798.