Update codegen example

[ntrel]

Update syntax for handling T.fields:

• Compiletime reflection v#43 (comment) - is syntax.
• Compiletime reflection v#43 (comment) - $if, $for.

Note: I haven't used $match because $if is the more fundamental feature which is needed anyway (and the design for [runtime] match isn't decided AFAIK).

[medvednikov]

Thanks, will be live a bit later today.