shared_vector implementation

Shared ownership of a single C array.

Tracks offset and length for each instance
allowing each owner to "view" a different
piece of the array.

Parts of shared_vector<T> which can work
for T=void are in shared_vector_base<T>.
Specializations shared_vector<void>
and shared_vector<const void> handle
un-typed arrays.

Allow casting to/from typed vector

Offsets and sizes of untyped vectors are tracked in
bytes.  Therefore casting to types where sizeof(T)>1
is undefined if the offset is not a multiple of
sizeof(T).

pvDataApp/misc/epicsException.h

@@ -103,7 +103,7 @@ namespace detail {
      *
      * Takes advantage of the requirement that all exception classes
      * must be copy constructable.  Of course this also requires
-     * the and extra copy be constructed...
+     * that an extra copy be constructed...
      */
     template<typename E>
     class ExceptionMixed : public E, public ExceptionMixin {