Revert to using size over num_elements in stan code

[seabbs]

#177 (fixed in #178) found a type instability when using size on M series Macbooks. This seems to have been fixed upstream but the fix isn't with users yet. When it is we should test and revert to using size() as the syntax is easier to pass.

[seabbs]

As reported by @sbfnk it looks like this isn't yet fixed upstream: #178 (comment)

[sbfnk]

Turns out the opposite is the case: with the upstream changes #178 no longer fixes #177 (because num_elements() now returns a long int).

[sbfnk]

Digging a bit deeper the offending line is

primarycensored/inst/stan/functions/primarycensored.stan

Line 41 in 2cfa802

array[4] int ids = {dist_id, primary_id, num_elements(params), num_elements(primary_params)};

and the issue is

Until C++11, narrowing conversions were permitted in aggregate initialization, but they are no longer allowed.

https://en.cppreference.com/w/cpp/language/aggregate_initialization

It worked previously with num_elements() when it happened to return int so that no type conversion was necessary.