#cfg in macros

[kaczmarczyck]

As noted in #127 , we should support #cfg inside macro calls, i.e. cbor_map.

[gendx]

See the register_structs! macro of Tock. It should boil down to matching $(#[$attr:meta])* before each item that can be #cfged.

[kaczmarczyck]

I'll mention this here since it is related:
rust-lang/rust#73663

It should be orthogonal though if we don't support #cfg in macros so far.