Add lagrange_basis function to Polynomial

[survived]

Adds Polynomial::lagrange_basis that calculates l_j(x):

There's similar method VerifiableSS::map_share_to_new_params that calculates lagrange basis too, but for x=0.

Since Polynomial::lagrange_basis generalises VerifiableSS::map_share_to_new_params, I'm going to deprecate the last one unless there are any objections (@omershlo ?) VerifiableSS::map_share_to_new_params uses Polynomial::lagrange_basis internally. No need to deprecate it.