perf(core): improve applyVariants method speed

[AndreyYolkin]

thanks

First of all, I want to thank you for the brilliant framework. While I interested of how it works I started looking into sources.

I found that applyVariants method could be improved a bit, but it helps to reach 7x speed supremacy over windicss:

• instead of chaining reduce() and reduce() methods we can simply write reduceRight(), as result will be the same (+ original array does not mutate)
• we may use variantHandlers.slice() instead of [...variantHandlers] as it 2x faster (according to jsbench.me).

I think there are a lot of other possibilities to improve the performance, but I think it's a nice start

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