delegatecall/callcode implementation

[yarikbratashchuk]

This issue tracks the implementation of delegate call in pallet-contract.

Delegate call can be used to:

• Create an upgradable contract via the Proxy pattern.
• Implement some library that can be called via delegate call.
• Split the logic of a big contract into small subcontracts.

There are different ways to implement delegated calls:

• Use the existing seal_call function and create a new PRESERVE_CONTEXT flag in CallFlags(possible implementation).
• Create a new seal_delegate_call with its own signature.

fn delegate_call(
    flags: CallFlags,  <------ The user can copy or forward input
    code_hash_ptr: u32, 
    input_data_ptr: u32,
    input_data_len: u32,
) -> Result<ExecReturnValue, ExecError>

This way we can keep API clean.