Fix precision of bigdecimal.power(integer, prec) #503

Job	Run time
ruby-versions / Generate Ruby versions	5s
ubuntu-latest 3.0	36s
ubuntu-latest head	31s
ubuntu-latest 2.7	1m 7s
ubuntu-latest 3.3	35s
ubuntu-latest debug	47s
ubuntu-latest 3.4	37s
ubuntu-latest 3.1	33s
ubuntu-latest 3.2	39s
macos-latest 3.2	27s
macos-latest 2.7	56s
macos-latest 3.1	32s
macos-14 3.2	27s
macos-latest debug	28s
macos-14 2.7	58s
macos-14 3.4	25s
windows-latest 3.3	2m 24s
macos-14 head	23s
macos-14 debug	29s
macos-14 3.3	22s
macos-14 3.1	30s
macos-latest 3.4	22s
macos-latest 3.3	24s
macos-latest 3.0	24s
macos-14 3.0	24s
macos-latest head	22s
windows-latest 2.7	3m 39s
windows-latest head	2m 12s
windows-latest mingw	2m 8s
windows-latest 3.4	2m 13s
windows-latest mswin	1m 29s
	27m 28s

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