Add the ability to save in _SCOPE_ field data of various types (not only account_name)

[s-medvedev]

Some smart-contracts in their tables use other types in _SCOPE_ field instead of account_name. The following contracts are known:

• eosio.token use symbol.name() as scope in stats table
• golos.control use symbol_type as scope in props table

In this case _SCOPE_ field in database looks like a meaningless string (for example: ........ehgp3, ........eha4b or ......2ndl3k3).

It is necessary to add the ability to specify _SCOPE_ field type in the ABI-description and use the conversion to/from this type when saving/reading data from the database.