Streamline Generation

[JasonBock]

I think I can clean up the incremental generator a bit...

• First, in IsSyntaxTargetForGeneration(), just look for AttributeSyntax nodes that have expected "names" like Map or MapAttribute (along with the other two options)
• Next, in TransformTargets()
  • Return (SyntaxNode node, INamedTypeSymbol source, INamedTypeSymbol destination, MappingContext context)?
  • If the given node is a MapToAttribute or MapFromAttribute, return the right values for that.
  • Else, if it's MapAttribute, return the right values.
  • Else, return null
• Finally, in CreateOutput()
  • Change nodes type to the tuple, and rename to targets
  • Remove the call to GetTargets() (and the method itself)
  • Note that I'll still have to keep the mappings dictionary.