Use `Num.bound` #5020

Job	Run time
rocq-elpi	57s
rocq-core	52s
coq	49s
stdlib	58s
coq-elpi	57s
mathcomp-bigenough	51s
mathcomp-finmap	50s
hierarchy-builder	52s
mathcomp	51s
mathcomp-classical	2m 54s
mathcomp-analysis-single	17m 14s
mathcomp-reals	4m 16s
mathcomp-analysis	15m 10s
mathcomp-experimental-reals	2m 0s
mathcomp-reals-stdlib	1m 41s
mathcomp-analysis-stdlib	16m 38s
	1h 7m 50s

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