make bn shareable when frozen #1226

Job	Run time
openssl-1.0.2u	50s
openssl-1.1.0l	53s
openssl-1.1.1w	54s
openssl-3.0.15	59s
openssl-3.1.7	59s
ruby-versions / Generate Ruby versions	3s
openssl-3.2.3	55s
openssl-3.3.2	55s
openssl-3.4.0	1m 3s
openssl-master	4m 10s
libressl-3.1.5	54s
libressl-3.2.7	59s
libressl-3.3.6	1m 1s
libressl-3.4.3	58s
libressl-3.5.3	59s
libressl-3.6.3	1m 0s
libressl-3.7.3	55s
libressl-3.8.4	56s
libressl-3.9.2	1m 3s
libressl-4.0.0	1m 0s
openssl-3.0.15 with fips provider	37s
openssl-3.1.7 with fips provider	39s
openssl-3.2.3 with fips provider	39s
openssl-3.3.2 with fips provider	40s
openssl-3.4.0 with fips provider	39s
openssl-master with fips provider	3m 31s
openssl-3.4.0 without legacy provider	1m 3s
ubuntu-22.04 2.7	59s
ubuntu-22.04 3.0	54s
ubuntu-22.04 3.1	1m 0s
ubuntu-22.04 3.2	1m 6s
ubuntu-22.04 3.3	1m 6s
ubuntu-22.04 head	59s
ubuntu-22.04 truffleruby	1m 28s
ubuntu-22.04 truffleruby-head	1m 42s
ubuntu-20.04 2.7	58s
ubuntu-20.04 3.0	1m 0s
ubuntu-20.04 3.1	1m 12s
ubuntu-20.04 3.2	58s
ubuntu-20.04 3.3	58s
ubuntu-20.04 head	59s
ubuntu-20.04 truffleruby	1m 28s
ubuntu-20.04 truffleruby-head	1m 49s
macos-latest 2.7	58s
macos-latest 3.0	37s
macos-latest 3.1	42s
macos-latest 3.2	48s
macos-latest 3.3	41s
macos-latest head	44s
macos-latest truffleruby	1m 1s
macos-latest truffleruby-head	1m 16s
windows-latest 2.7	6m 18s
windows-latest 3.0	4m 12s
windows-latest 3.1	4m 9s
windows-latest 3.2	4m 17s
windows-latest 3.3	5m 17s
windows-latest head	4m 22s
windows-latest ucrt	4m 38s
windows-latest mswin	3m 30s
	1h 31m 20s

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