allow to combine more than two blocks

this MR allows to give any number of blocks to recode them. this is a convenience to avoid combining the blocks pair-wise and create intermediate and useless blocks, e.g. by defining the following Nushell command with the komodo.nu module

def "komodo full-recode" []: list<string> -> string {
    let blocks = $in
    match ($blocks | length) {
        0 => { return null },
        1 => { return $blocks.0 },
    }

    $blocks | skip 1 | reduce --fold $blocks.0 {|it, acc| komodo combine $it $acc}
}

one can now do directly

komodo combine ...(komodo ls)

which will create a single new fully recoded block!

changelog

• new fec::combine that takes a list of shards and their coefficients and combines them, returns None if the slices are empty or not of the same length

pub(super) combine<E: Pairing>(
    shards: &[Shard<E>],
    coeffs: &[E::ScalarField],
) -> Option<Shard<E>>

• modified recode that takes any number of blocks and returns an Option if there is none

pub recode<E: Pairing>(blocks: &[Block<E>]) -> Result<Option<Block<E>>, KomodoError>

• the komodo combine command from komodo.nu can now take any number of blocks, even 0 by giving a nice error