Fix span for conflicting values

Added

• IdentValue to keep hold of both value and the source ident

Changed

• Breaking Change: ConvertParsed::aggregate() now takes/returns
  IdentValue
• Improved span for conflicting values

The big refactor

Changed

• Breaking Change: Moved some syn types behind feature full
• Breaking Change: Refactored attributes
• Use interpolator for error messages
• Breaking Change: Compile time verify if ident is given through helper
  trait