Field visibility

[LegNeato]

You cannot set field visibility currently.

I have added it in my branch @ https://github.com/LegNeato/codegen/tree/issue-3-and-4-field-documentation-annotation and will put up a PR when https://github.com/carllerche/codegen/pull/6 lands.

[gorup]

I got around this by having the name of the field have the word pub in the field name, e.g.

s.field("pub num", "usize");

But yeah, +1 to having this be supported!