Fix emit inconsitencies and add tests

[Sh3Rm4n]

While trying to convert the parsed payload of information elements into the binary format, I've noticed inconsistencies regarding some emit implementations.

I've fixed some of them (which I needed) mostly based on the assumption, that the Parsable implementation is correct.
On exception is the Nl80211RateAndSelector implementation, where the comments from this type and Nl80211Element::SupportedRatesAndSelectors where already inconsistent.

Also I've added some roudtrip test, using macros to reduce the repetition. I thought about adding more tests, but I wanted to propose this as a starting point for now.