make bn shareable when frozen #1195

Job	Run time
openssl-1.0.2u	49s
openssl-1.1.0l	52s
openssl-1.1.1w	1m 2s
ruby-versions / Generate Ruby versions	3s
openssl-3.0.15	1m 7s
openssl-3.1.7	1m 3s
openssl-3.2.3	1m 1s
openssl-3.3.2	1m 0s
openssl-master	3m 49s
libressl-3.1.5	57s
libressl-3.2.7	58s
libressl-3.3.6	1m 1s
libressl-3.4.3	58s
libressl-3.5.3	59s
libressl-3.6.3	1m 3s
libressl-3.7.3	1m 4s
libressl-3.8.4	59s
libressl-3.9.2	1m 9s
openssl-3.0.15 with fips provider	45s
openssl-3.1.7 with fips provider	40s
openssl-3.2.3 with fips provider	46s
openssl-3.3.2 with fips provider	39s
openssl-master with fips provider	3m 31s
openssl-3.3.2 without legacy provider	1m 9s
ubuntu-22.04 2.7	1m 4s
ubuntu-22.04 3.0	53s
ubuntu-22.04 3.1	1m 8s
ubuntu-22.04 3.2	59s
ubuntu-22.04 3.3	1m 3s
ubuntu-22.04 head	1m 3s
ubuntu-22.04 truffleruby	1m 31s
ubuntu-22.04 truffleruby-head	1m 52s
ubuntu-20.04 2.7	1m 4s
ubuntu-20.04 3.0	1m 2s
ubuntu-20.04 3.1	57s
ubuntu-20.04 3.2	1m 13s
ubuntu-20.04 3.3	55s
ubuntu-20.04 head	1m 2s
ubuntu-20.04 truffleruby	1m 29s
ubuntu-20.04 truffleruby-head	1m 42s
macos-latest 2.7	50s
macos-latest 3.0	54s
macos-latest 3.1	57s
macos-latest 3.2	53s
macos-latest 3.3	50s
macos-latest head	40s
macos-latest truffleruby	1m 16s
macos-latest truffleruby-head	1m 45s
windows-latest 2.7	6m 2s
windows-latest 3.0	4m 20s
windows-latest 3.1	4m 36s
windows-latest 3.2	4m 26s
windows-latest 3.3	3m 25s
windows-latest head	4m 23s
windows-latest ucrt	4m 19s
windows-latest mswin	2m 21s
	1h 28m 18s

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