v0.5.1

• fixed broken AVX-512 support for vector indexing (#16)
• improved AVX-512 support for logical indexing
• new public (non-exported) types AbstractFixedOrSmallVector, AbstractFixedOrSmallOrPackedVector
• new faster methods for vectors with hardware-type elements:
  • duplicate and duplicate! for AbstractSmallVector
  • popfirst, popat for AbstractFixedVector
• added duplicate for AbstractFixedVector
• faster getindex for PackedVector for x86 architectures