PMVector sum is extremely slow

[AtharvaKhare]

Here is the comparison with array:

a := ((1 to: 5000) collect: [ :i | i ]) asPMVector.
a sum. "Best of 10 runs: 568ms"

b := ((1 to: 25000000) collect: [ :i | i ]).
b sum. "Worst of 10 runs: 551ms"

This means that PMVector sum is at least 5,000 times slower than Collection sum.

Possible fix: Don't override sum method. Is there any specific reason to do so?

[SergeStinckwich]

I try to remove PMVector>>sumand it run much faster. The tests are all green.
I think we can remove it.