v0.8.0-preview

• Contents of grown memory is now initialized to 0 before being made available for use.
  Special thanks to @TYoung86 for identifying and fixing this issue.
• Memory can now grow from an initial size of zero.
• Improved disposal tracking of UnmanagedMemory.
  • Previously it was possible to un-dispose objects of this type by calling Grow.
    An object revived in this way would leak memory if it was not explicitly Disposed before being discarded.
• Added .NET Standard 2.1 build with no dependencies.