Expose sort on Vector

[VeaaC]

Often flatdata data needs to be sorted. By not exposing sort methods on Vector the user has to sort another structure first before serializing. This needs special wrapper since the Vector classes just expose proxies / proxy iterators.

This is distinct from external memory sort on ExternalVector: All the data is in memory.

FYI: @boxdot

[VeaaC]

After #169 a Vector can be cast to a mutable slice if (and only if) it does not contain ranges. Those slices should then be sortable via normal methods.

Removing Rust label