Add tests for various size limits for data types

[anton-trunov]

For instance, the docs say:

As the upper bound of the smart contract state size is around 65,000 items of type Cell, it constrains the storage limit of maps to be about 30,000 key-value pairs for the whole contract.

We should test doc statements like the above.

So, we need tests for at least the following items:

• maximum map size
• maximum cell number
• maximum number of struct fields