Important performance degradation on XOR benchmark

[mratsim]

I've noticed an important performance degradation on the xor benchmark: 26 seconds on single threaded while it was 10 seconds before when investigating #141 and after introducing #143:

Bisect can be started from d553685

Some Nim changes may also be involved but the degradation seems too high for that.

[mratsim]

First investigation shows that the issue is not in Arraymancer but in Nim.

Nim 0.17.3 with tag 3174cfe55c6da6c83dfe83667744ccd8acb7a1ce bootstrapped with csources from 0.17.2 (08c310bb8277611711ca06619526ec08f1feffaf) is fine on Arraymancer 69f3c32#diff-b6f5223b568dfd995ddc1648fbb74ba4

But the very latest compiler gives 30s (d62fe7d5384139ad55277c108bf88c105a3c5d30)

And the one i used to first report (855956bf617f68ac0be3717329e9e1181e5dc0c6) gave 27~40 seconds depending on the Arraymancer commit:

[mratsim]

Performance regression was introduced by the following commit: nim-lang/Nim@358709e, raised upstream: nim-lang/Nim#7743

[mratsim]

Fixed by nim-lang/Nim@88cf657

It is even 30% faster now :O