Consider if fragmentation is an issue

[edsko]

As of #154 we allocate all structs that in the C API are passed or returned by value as pinned bytearrays. This could result in memory fragmentation, if any of these values are long-lived. If this is an issue, we may decide to use unpinned bytearrays for some of these, at the cost of having to do more copying for each C API call.