Fast implementation of Hamming weight

The target method: System.Reflection.Internal.BitArithmetic.CountBits
Old implementation uses 24 arithmetic operations
New fast implementation uses 12 arithmetic operations
Limit application of the JitIntrinsicAttribute

This attribute should limit where it can be placed to something that the
JIT will respect.
Merge pull request #169 from briangru/master

Limit application of the JitIntrinsicAttribute
Adding badges for pull request and issue times.
Merge pull request #173 from Petermarcu/adding_badges

Adding badges for pull request and issue times.
Merge pull request #155 from AndreyAkinshin/FastHammingWeight

Fast implementation of Hamming weight