Another addition to stl_container_of_vex_vector

Got the reason for the issue #25:
    Each instance of vex::vector<> inside stl container stays correct,
    but the actual OpenCL buffers may be reallocated in the process.

The outcome: it is safe to store vex::vectors inside stl containers,
but one can not assume that OpenCL buffers will stay the same.

@neapel, that was the reason behind errors you got when you tried to
store vex::vectors instead of cl::Buffers in fft/plan.hpp.