Compile-time type reflection API?

[StasB]

I know it's possible to get information by examining the AST associated with a 
type's symbol, but wouldn't it make more sense to provide a compile-time API 
that allows one to query it directly? For instance, something like 
`TypeName.hasField("fieldName")` or `for field in TypeName.fields` would be a 
lot more robust and easy to use than manually scanning the AST, and probably 
more efficient (I assume the compiler already keeps such information somewhere 
in a form that's efficient to query).