Fix unqualified swap for small_vector

[monerinngu0]

Unqualified swap(a, b) for two small_vector objects was resolved
to the hidden friend swap of the underlying vector. This caused
pointers to internal storage to be treated as dynamically allocated
storage and deallocated during destruction.

Add a hidden friend swap to small_vector that forwards to its
capacity-aware member swap.

A regression test verifies unqualified ADL swap when both vectors use
their internal storage.

Fixes #342.

[igaztanaga]

Many thanks for the fix!