change storage keys serde serialization to their string form

[tzemanovic]

Serde based serialization of the storage keys uses the inner segments, which is not very readable in e.g. JSON format (in for example tx result):

"changed_keys": [
        {
          "segments": [
            {
              "AddressSeg": "tnam1pyqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqej6juv"
            },
            {
              "AddressSeg": "tnam1q9gr66cvu4hrzm0sd5kmlnjje82gs3xlfg3v6nu7"
            },
            {
              "StringSeg": "balance"
            },
            {
              "AddressSeg": "tnam1qgqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqc8j2fp"
            }
          ]
        },
...

instead, it should just be:

"changed_keys": [
    "#tnam1pyqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqej6juv/#tnam1q9gr66cvu4hrzm0sd5kmlnjje82gs3xlfg3v6nu7/balance/#tnam1qgqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqc8j2fp",
...