Add DigestAs functionality

[survived]

Add functionality to digest a field "as another type". E.g. hash map can be digested "as btree map" (implementation will convert hash map to btree map at runtime).

Other example is when we have Option<Vec<u8>>, which may be digested as Option<udigest::Bytes> which will encode vector as bytes (by default, it is digested as a list of u8, which results in a huge overhead)

[survived]

Updated all the deps:

• Use #[udigest(as = ...)] attribute cggmp21#106
• Use #[udigest(as = ...)] attribute paillier-zk#51
• Fix no_std support after updating to latest udigest generic-ec#43