improve serialize std::vector sdsl code

[mpetri]

some sdsl data structures use std::vectors to store data. currently there are several helper functions that can be used to serialize and load the content stored in these vectors. unfortunately, the current way this is implemented is very slow as each element in the vector is serialised individually. there might be a faster way to do this for POD.

[simongog]

This is a performance enhancement and not essential for the v2 release. But we will implement it later.