feat: improve add performance

[orisano]

I improved Add performance.

name         old time/op  new time/op  delta
AddSafe-8     155µs ± 1%   137µs ± 1%  -11.73%  (p=0.000 n=9+10)
AddUnsafe-8   145µs ± 3%   125µs ± 1%  -13.87%  (p=0.000 n=10+9)

[jeffwidman]

This was closed because of the branch change from master to main. If this PR is still relevant, please re-open a PR with this code against main.

[jeffwidman]

@orisano if these benchmarks are indeed still relevant against the new generics-based implementation, then this looks like a useful PR... 11%-13% improvement is nothing to sneeze at!

[jeffwidman]

Re-opened / merged as #88