Use const generics

[tbrezot]

• use constant generics instead of generic arrays:
  • KEY_LENGTH had to be added to the Dem trait to be able to use { Aes256GcmCrypto::KEY_LENGTH } as a constant generic
• remove SymmetricCrypto trait -> Dem becomes the default trait for symmetric encryption
• add DhKeyPair trait, which represent an asymmetric key pair based on a space where the computational Diffie-Helman is intractable
• remove block.rs: checking the plaintext/ciphertext size to prevent nonce reuse should be done in the DEM implementation as it is specific to nonce-based symmetric ciphers; block structure is pushed up onto the call stack through the use of associated data
• implement NonceTrait::increment() without BigInt dependency and remove big-int from the crate dependencies
  • is this method really needed since we will most likely let the symmetric cipher crate manage incrementing the nonce ?
• remove std where core can be used
  • the only place where std is needed is in aes_256_gcm_pure.rs
  • as a future work, we could try to use aes-gcm with the no-alloc feature

Edit: there still is a std dependency as a rand_core feature

Edit: after discussion with @bgrieder I removed increment() from the nonce trait