This task is a followup to https://bugs.chromium.org/p/chromium/issues/detail?id=671759 and
https://bugs.chromium.org/p/chromium/issues/detail?id=682215

Container can have some memory inside, where it can store up to certain amount of elements. This is a common optimisation, used in many implementations of std::string for example.

For flat_set it can be useful, because:
* - flat_sets are good for small data sets, this is where small buffer optimisation may fire.
* - when doing set operations, it's not unreasonable to store flat_sets in a vector or in another flat_set. Then small buffer optimisation can get all the elements in one contiguous which can be of benefit.

A possible implementation would be to write a small_vector much like llvm's one:
http://llvm.org/docs/doxygen/html/classllvm_1_1SmallVector.html and then use it in flat_set.