Add missing derive(Getters) for dkg::{round1, round2}

[StackOverflowExcept1on]

Resolves #737

[StackOverflowExcept1on]

@conradoplg could you take a look? real use cases:

1. smart contract has in its storage round1_packages: BTreeMap<Identifier<C>, round1::Package<C>> and you as a participant want to remove your identifier from round1_packages: https://github.com/StackOverflowExcept1on/roast/blob/c31716b0eeefcaf6ab7029e7bb882334133c07c2/roast-core/src/dkg.rs#L311
2. participant wants to execute dkg::part3(), but with a small difference - bad participants will be recorded in round2_culprits_set, and then passed to smart contract, so that it will apply sanctions to them (for example, take away deposit of bad DKG participants): https://github.com/StackOverflowExcept1on/roast/blob/c31716b0eeefcaf6ab7029e7bb882334133c07c2/roast-core/src/dkg.rs#L362

[conradoplg]

Thanks! One question: why did you skip the coefficients field in particular?

[StackOverflowExcept1on]

I skipped the coefficients field because there is already a method that returns &[T] instead of &Vec<T>, which is a more correct type (see internal method pub fn coefficients(&self) -> &[Scalar<C>]). Unfortunately, in derive getters you can't choose a more correct type (e.g. u16 instead of &16).

[conradoplg]

Ah of course, thanks