Vectorise BitArray (dotnet/corefx#41896)

Vectorise BitArray (dotnet/corefx#41896)

* Speed up BitArray

1. Use AVX2, if available, for And/Or/Xor
2. Vectorize Not
3. Use Span<T>.Fill() for SetAll()
4. Add more test sizes to account for And/Or/Xor/Not loop unrolling cases

* Vectorise BitArray(bool[])

* Vectorise BitArray further

* Vectorise CopyTo(Array, int) when copying to a bool[]

* Use Vector128/256.Create and store it in static readonly field instead of loading from PE header

* Add some random test data for BitArray

Commit migrated from https://github.com/dotnet/corefx/commit/a4f04476ea74c61a52168cf0b31c0281822ecad3