Resolve some todos in code

[pv42]

There are some //TODOs in the code that can be removed

//TODO verify this is no longer necessary since we're supporting both mavlink1 and mavlink2; //TODO verify that id_width of u8 is OK even in mavlink v1:

• We have an explicit error MessageWriteError::MAVLink2Only that check for such cases
• There is no functional way to generate only MAVLink1 messages right now

// TODO: these lints are produced on a couple of cubepilot messages (..)

• the cubepilot dialect is fixed
• unclosed tags should be caught by quickxml, unless selfclosing tags are meant, with are now supported
• added an assert that fails when a message would have a 0 bytes payload (this is already forbidden since messages must have at least one field)

//TODO check in MavFrame::ser()

• use a checked int conversion instead, since the function can already panic on incorrect usage that should be fine
• using the uncheck cast would result in incorect bytes streams when it overflows