feat: wrap public key in a class

web3.js/src/index.js

@@ -1,6 +1,7 @@
 // @flow
 export {Account} from './account';
-export {Connection} from './connection';
-export {Transaction} from './transaction';
-export {SystemProgram} from './system-program';
 export {BudgetProgram} from './budget-program';
+export {Connection} from './connection';
+export {PublicKey} from './publickey';
+export {SystemProgram} from './system-program';
+export {Transaction} from './transaction';