Generate From<> rust conversions when the encoding is infallible

[hchataing]

When converting from a child packet to a parent the encoding is often infallible, but the rust backend still generates TryFrom<> implementations instead of From<>.

For example:

packet Command {
   op_code: 8,
   _size_(_payload_),
   _payload_,
}

packet OneCommand : Command (op_code = 1) {
   x: 16,
   y: 16,
}

Generates

impl TryFrom<OneCommand> for Command {}

When the conversion can never fail.