Add accuracy and grade

[uzervlad]

I assume calculating accuracy should be faster than reading from memory

[MaxOhn]

One downside of calculating things manually is that it likely increases the final binary size a little. Can't say by how much but it should maybe be checked and factored into the decision whether this should be merged.

Another big factor for the decision is of course the performance difference so that could be benchmarked.

[486c]

Calculating acc & grade by ourself gonna be faster than any syscall to OS, especially when we talking about linux.
@uzervlad could you rebase/merge newer changes from master + add trace spans, if you don't mind. if not i'm gonna do it myself :)