feat(soroban): Support structs

[salaheldinsoliman]

This PR brings support to user defined structs in Soroban's Solidity.

Here are some points to note:

• In-memory structs were naturally supported by integrating a sorobanVm-friendly bump allocator that was introduced in Soroban: support dynamic memory arrays #1838

• For storing structs in storage, we do as follows:
  We encode struct fields as Soroban Vals, and store each one separately in storage where the key of each element is SorobanVec[storage_slot, field_index].

When we retrieve a whole struct, i.e StructDef memory my_struct = my_structs_map[my_struct], we simply loop over all fields, fetch them from storage, and create an in memory instance.

That is why we strongly advise to access a single struct field at a time instead of fetching it all from storage at once. This a complete example:

contract timelock {
    
    mapping(address => TimeLock) timelocks;
    
    struct TimeLock {
        uint64 release_time;
        address beneficiary;
        uint64 amount;
    }

   

    function create_timelock(
        uint64 release_time,
        address beneficiary,
        uint64 amount
    ) public returns (uint64) {

        TimeLock memory tl = TimeLock({
            release_time: release_time,
            beneficiary: beneficiary,
            amount: amount
        });

        timelocks[beneficiary] = tl;

        return tl.amount;
    }

    function get_timelock_amount_cheap_op(address beneficiary) public view returns (uint64) {
      return timelocks[beneficiary].amount;
    }

    function get_timelock_amount_expensive_op(address beneficiary) public view returns (uint64) {
      TimeLock memory tl = timelocks[beneficiary];
      return tl.amount;
    }
}

Here, get_timelock_amount_expensive_op not only retrieves all fields from storage, but puts in extra instructions construct the struct in-memory.

get_timelock_amount_cheap_op on the other hand simply calls the soroban host function get_contract_data and doesn't need extra work.