Completely implement annotations

[dahlia]

Continued from #34, we need to implement annotations for all sorts of declarations, including:

• Boxed type
• Enum type
• Enum member
• Record type
  • Record field
• Union type
  • Union tag
    • Union field
• Type alias
• Service (Partially implement annotations #34)
  • Service method (Partially implement annotations #34)
    • Method parameter
• Import name (not an import line)

Note that last part. Each import line is represented as multiple Import types in AST, we currently have no easy way to annotate per import line.