Add bitset support in abi.

[greg7mdp]

Resolves #337.

Example contract with hi action taking a bitset:

#include <eosio/eosio.hpp>
#include <eosio/bitset.hpp>

class [[eosio::contract]] hello : public eosio::contract {
public:
   using eosio::contract::contract;
   [[eosio::action]] void hi(eosio::name user, eosio::bitset b) {
      print( "Hello, ", user, ", --> ", b);
   }
};

calling hi action

❯ ./bin/cleos push action hello hi '["bob", "0010"]' -p hello@active                                                        
executed transaction: d7084775dcd3b93c6ff44f74db838fe140909e3cc8a70b8677d431164de4c1a3  104 bytes  320 us
#         hello <= hello::hi                    {"user":"bob","b":"0010"}
>> Hello, bob, --> 0010
warning: transaction executed locally, but may not be confirmed by the network yet         ] 

[spoonincode]

The output ABI version needs to be updated. Did we make a conclusive decision on 1.3 vs 2.0 🥴 ? idk if we should (or if it's worth) trying to detect if any 1.3/2.0 features are being used and only output the new version in that case.

[spoonincode]

Still need to determine what to do about generated ABI version.

[spoonincode]

ABI versioning stuff will be handled separately in #355