[Meta] List of Changes in FlatSharp 6

[James Courtney]

This is a tracking item to list the changes in Flatsharp 6.0.0.

Breaking changes:

• ISharedStringReader has been removed.
• FlatBufferItem.CustomGetter has been removed.
• ArrayInputBuffer is now a struct
• MemoryInputBuffer is now a struct
• ReadOnlyMemoryInputBuffer is now a struct
• SharedStringReaderFactory has been removed from IInputBuffer.
• ArrayInputBuffer.Wrapper has been removed
• MemoryInputBuffer.Wrapper has been removed
• ReadOnlyMemoryInputBuffer.Wrapper has been removed.
• IFlatBufferAddressableStruct has been removed.
• SpanWriter.Instance has been removed. Use default(SpanWriter) instead.
• ArrayInputBuffer has been split into ArrayInputBuffer and ArraySegmentInputBuffer
• The fs_fileId attribute has been dropped.
• Unquoted metadata no longer supported. Compiler now runs through flatc, so attribute declarations are now necessary.
• The FlatSharp.Compiler Package no longer targets .NET Framework or .NET Core 2.1 (You can still use it to build code that will run in these frameworks, though!)
• ISharedStringWriter has been changed. The PrepareWrite method has been renamed Reset, and there is a new flag called IsDirty that is expected to be true if the shared string writer has any pending strings.

Other changes

• FlatBufferSerializer exposes some additional methods that allow avoiding boxing of IInputBuffer instances.
• ArrayInputBuffer now ~10% faster as a result of the split above.
• WriteThrough now supported for value structs
• Serializers now have a default instance of SharedStringWriter with a hash table capacity of about 1K items.

Major changes

• Compiler is being completely rewritten to be based on bfbs output from flatc.
• PropertyCache, VectorCache, and VectorCacheMutable are being collapsed into Progressive deserialization mode.
• The SharedString type has been removed and replaced with an annotation. No more SharedString type muddling things!
• Unions are now value types
• The Flatsharp.Unsafe package has been removed.