Improve dynamic events type sizes API

[ascjones]

Previously if any primitive type for an Event didn't have its type size registered it would fail. This PR relaxes that constraint, but it will still fail if we receive one of those Events with a missing type size registration. To address this, the API no allows registering of type sizes by the user, so the library does not need to be updated to have all primitive types registered.

New API:

xt.watch()
    .events_decoder(|decoder| {
    decoder.register_type_size::<(u64, u64)>("IdentificationTuple")
})
.submit(balances::transfer::<Runtime>(dest.clone().into(), 10_000))

The refactoring here should make it easier to add back static event decoding (#28)