Change Measurement ID type from `std::size_t` to `unsigned int`

[beomki-yeo]

The measurement Ids are accessed frequently in the greedy resolver, and I think its current type (std::size_t) is making the algorithm slow a bit.
I suggest changing its type to unsigned int as 64bit seems definitively an overkill?

@krasznaa @stephenswat how do you guys think?

[krasznaa]

In general, I don't like using std::size_t in device code. So even for a try, I'm on board.

Once, after converting the fitted track EDM, I finally get to replace traccc::measurement with an SoA, I was definitely going to change this variable into an unsigned int. Since you're right, we'll never need 64 bits to give unique IDs to the measurements. We just need 32. I mean, 16 bits seems almost enough. But not quite...

[beomki-yeo]

Awesome. Then I will assign you for this task 😃
16 bits may not be enough though as I see more than 10^5 measurements from ttbar event