improve abi generation

[dimas1185]

fixes #225

after #201 we introduced a new logic where structure defined out of contract doesn't go to ABI.
There is an exception to that that is actively used by system contract. A lot of structures are defined out of contract but those have [[eosio::contract("name")]] tag. That case was added to exceptions and also search of contract class was improved as in some cases in reference-contracts it was not found the old way. Also a fix added where structure that was defined out of contract was not properly defined in structures. That is reflected in singleton_contract.abi and corresponding test.

Change Description

API Changes

• API Changes

Documentation Additions

• Documentation Additions

[heifner]

Please add a better PR description.

[greg7mdp]

@dimas1185 I'm sorry I don't know enough to review this.

[ericpassmore]

Note:start
group: STABILITY
category: BUG
summary: Backward compatible for ABI generation in older system contracts by including structures defined in an extended context.
Note:end