make bn shareable when frozen #1192

Job	Run time
openssl-1.0.2u	49s
openssl-1.1.0l	53s
openssl-1.1.1w	54s
openssl-3.0.15	55s
ruby-versions / Generate Ruby versions	2s
openssl-3.1.7	55s
openssl-3.2.3	54s
openssl-3.3.2	1m 0s
openssl-master	3m 44s
libressl-3.1.5	55s
libressl-3.2.7	1m 4s
libressl-3.3.6	57s
libressl-3.4.3	1m 4s
libressl-3.5.3	58s
libressl-3.6.3	59s
libressl-3.7.3	56s
libressl-3.8.4	56s
libressl-3.9.2	57s
openssl-3.0.15 with fips provider	40s
openssl-3.1.7 with fips provider	42s
openssl-3.2.3 with fips provider	38s
openssl-3.3.2 with fips provider	40s
openssl-master with fips provider	3m 20s
openssl-3.3.2 without legacy provider	1m 3s
ubuntu-22.04 2.7	1m 1s
ubuntu-22.04 3.0	58s
ubuntu-22.04 3.1	1m 2s
ubuntu-22.04 3.2	1m 2s
ubuntu-22.04 3.3	57s
ubuntu-22.04 head	57s
ubuntu-22.04 truffleruby	1m 30s
ubuntu-22.04 truffleruby-head	1m 40s
ubuntu-20.04 2.7	1m 0s
ubuntu-20.04 3.0	58s
ubuntu-20.04 3.1	1m 0s
ubuntu-20.04 3.2	1m 1s
ubuntu-20.04 3.3	55s
ubuntu-20.04 head	56s
ubuntu-20.04 truffleruby	1m 30s
ubuntu-20.04 truffleruby-head	1m 42s
macos-latest 2.7	49s
macos-latest 3.0	37s
macos-latest 3.1	40s
macos-latest 3.2	38s
macos-latest 3.3	41s
macos-latest head	47s
macos-latest truffleruby	56s
macos-latest truffleruby-head	59s
windows-latest 2.7	5m 32s
windows-latest 3.0	4m 1s
windows-latest 3.1	4m 12s
windows-latest 3.2	3m 54s
windows-latest 3.3	6m 48s
windows-latest head	3m 56s
windows-latest ucrt	3m 33s
windows-latest mswin	3m 4s
	1h 25m 11s

Provide feedback