Refactor {:borsh, Module} serialiser and deserialiser

[alexfilatov]

WHY

Currently, when we find {:borsh, PublicKey} we treat it as a Borsh structure and serialize/deserialize according to the scheme inside that structure.
I believe we can even more simplify, and create scheme like this e.g:

old schema:

use Borsh,
      schema: [
        signer_id: :string,
        public_key: {:borsh, PublicKey},
        nonce: :u64,
        receiver_id: :string,
        block_hash: [32],
        actions: [
          {:borsh, ActionOne}, 
          {:borsh, ActionTwo}
        ]
      ]

new improved schema:

use Borsh,
      schema: [
        signer_id: :string,
        public_key: PublicKey,
        nonce: :u64,
        receiver_id: :string,
        block_hash: [32],
        actions: [
          ActionOne, 
          ActionTwo
        ]
      ]

WHAT

so if the value is just a module - treat this as a module with borsh schema.

IMPORTANT

Allow backward compatibility with the {:borsh, ActionOne}