IBC: refactor for typed storage

[cwgoes]

We should create helper functions to ensure that we never write values of the wrong type to particular storage keys, and we should avoid mingling code which handles logic and code which handles serialisation.

For example (from shared/src/ledger/ibc/mod.rs)

    // the client counter
    let key = client_counter_key();
    let value = 0_u64.to_be_bytes().to_vec();
    storage
        .write_bytes(&key, value)
        .expect("Unable to write the initial client counter");

This should be refactored into (in some appropriate file):

pub fn write_client_counter(value: u64) {
    let key = client_counter_key();
    let value = value.to_be_bytes().to_vec();
    storage
        .write_bytes(&key, value.to_be_bytes())
        .expect("Unable to write the client counter");
}

such that the original code needs to be only

ibc_storage::write_client_counter(0_u64);

Does this make sense? We should apply this pattern generally, starting with the IBC code.