Hold a reference to byte arrays when serializing to bech32

Hold a reference to byte arrays when serializing to bech32

When we serialize from a byte array to bech32 in
`lightning-invoice`, we can either copy the array itself into the
iterator or hold a reference to the array and iterate through that.

In aa2f6b47df312f026213d0ceaaff20ffe955c377 we opted to copy the
array into the iterator, which is fine for the current array sizes
we're working with, but does result in additional memory on the
stack if, in the future, we end up writing large arrays.

Instead, here, we switch to using the slice serialization code when
writing arrays, (very marginally) reducing code size and reducing
stack usage.