Feature/action results

[larryk85]

Change Description

Add support for ABI generation and auto code generation for action results.

API Changes

• API Changes

eosio actions can now have a return type other than void.

Documentation Additions

• Documentation Additions

eosio actions can now be defined as follows.

[[eosio::action]]
int action1() {
   ...
   return 42;
}
[[eosio::action]]
std::string action2() {
  std::string default_string_value = "action2 action";
   ...
   if (something)
      return "foo";
   ...
   return default_string_value;
}

This will now generate the correct code to call set_action_return_value and populate the fields for action_results in the ABI.

This PR also introduces a new flag for the compiler --abi-version this can be given to generate an ABI that is conformant with currently either v1.1 or v1.2.