Platform Support Changes & Improved Error Handling

• Removed WinRT support.
• Added NetStandard 2.0 support.
• Improved error handling during item reinitialisation - an event is now raised to notify of exceptions thrown by the reinitialisation callback as the exception cannot be caught by the host process if it occurs during background reinitialisation.

Improved Performance & Reduced Allocations

Breaking change: New ErrorOnIncorrectUsage property on PoolPolicy can be used to enable duplicate add detection but is off by default because it hurts performance and allocations. Should only be enabled on debug/test builds when looking for problems. Old behaviour was to always check for duplicate adds and ignore them. New options are don't check (default, best perf), or check and throw InvalidOperationException to indicate incorrect usage (use only in debug/test builds).