small_vector cannot go back to use stack space

[igaztanaga]

As reported in:

https://stackoverflow.com/questions/54429138/boost-small-vector-cannot-go-back-to-use-stack-space

small vector does not go back to the internal storage even after shrink_to_fit. This behavior is explicitly documented for the class but maybe it should be revisited, specially for shrink_to_fit.

The LLVM implementation does not seem to offer this feature, Folly seems to implement construct + swap to achieve it in shrink_to_fit.

In the same line, in current swap implementation, when one vector uses a dynamic buffer and the other one the internal buffer, swap allocates also a dynamic buffer for the vector with the internal buffer. This should be avoided

[igaztanaga]

The commit aa35950 finishes changes in small_vector so that:

• shrink_to_fit migrates data to the internal buffer if size() <= internal_capacity().
• swap acquires the existing dynamic buffer to avoid moving elements, and uses the internal buffer if the dynamic buffer can't be acquired and data fits in the internal buffer.