move HltDAQ/src/component/pun.h to Kernel/pun.h

In order to use one single 'type punning' solution, move pun.h from the HltDAQ component library into LHCbKernel.

Example usage:

int i = ... ;
float f = pun_to<float>( i );

this results in a float which is bitwise identical to the original int (but of course has a different numerical value).