Encoding of interpreter-level script representation

[foxcpp]

Sieve scripts might be loaded many times again and again while processing incoming messages. While it is possible to store interp.Script objects in memory, a practical implementation might want to avoid doing so as the size and amount of scripts can be significant. One potential solution is to define an efficient encoding of interp.Script data so it can be quickly restored from external cache on-demand without the cost of parsing the original Sieve code.