MSVC workaround for broken `using detail::_` warning

Current MSVC generates totally bizarre errors:

    error C2884: 'pybind11::detail::_': introduced by using-declaration
    conflicts with local function 'pybind11::detail::_'

which makes no sense (since the supposed "conflict" is the function
itself).  Work around it by `using namespace detail;` instead (which
also lets us drop a bunch of other `detail::` qualifications, so isn't
actually a bad thing).