Make `ConstAccessorSimple` safe

[JamesMcClung]

The following is currently unsafe:

int p = 0; // patch index
auto prts = mprts.accessor()[p]; // unsafe

// <- result of `mprts.accessor()` may be dropped here

prts.size() // seg fault if drop happened

The above seg fault happened in #354, and was ostensibly fixed in #355, but that fix was incompatible with CUDA.

For now, the safe alternative is:

auto acc = mprts.accessor();
auto prts = acc[p];
// safe to do whatever