Big Refactor

[jamescourtney]

This is an attempt to remove the spaghetti that the FlatSharp serializer had become. Key parts of the change:

• Code gen is moved into the type model
• Type model can be easily extended to support more things in the future
• Type-testing (ie, if foo is Table) replaced by feature testing (if foo.CanBeInVector)
• Support for custom types via an ITypeModel provider implementation (not yet implemented for the compiler)