Turns out that Enablers as typically used for
member functions don't work to select
constructors.
Move this selection logic to
struct detail::static_shared_vector_caster<>
to correctly allow no-op casts (eg. void to void).
Previously this would not compile.
Allows PVScalarArray getAs() and putFrom() using
shared_vector<const void>.
Call when appropriate (putFrom(), copyIn(), and replace()).
Not called by swap(), take(), reuse(), or shareData().
Users of the second set of methods are expected to call
one of the methods in the first set, or call postPut() directly.
Document when postPut is (not) called.
