encode_to should return Result

[arkpar]

In case provided slice is not big enough it should return an error, rather than panic.

[gui1117]

Some solutions:
1 - we make Output returning a result and add the method encode_to_vec in Encode trait so ppl can make use of a trait as an Output without having to call expect on the result.

2 - we generalize first solution by having Output and FailableOutput, and there is two method in Encode which are encode_to, and failable_encode_to (we should come up with better names) and encode_to as default implementation which calls failable_encode_to (because FailableOutput will be implemented for Output) and unwrap the result.

3 - same as 1 but we don't event introduce encode_to_vec, but then all current code that uses this method will have to expect the result with the proof: "Vec implementation of Output never returns error".

I would go for 1

[gui1117]

also note that currently Output is only implemented for Vec<u8> in no_std