Make fbuffer_inc_capa easier to inline #94

Job	Run time
ruby-versions / Generate Ruby versions	2s
ubuntu-22.04 2.7	1m 8s
ubuntu-22.04 3.0	31s
ubuntu-22.04 3.1	42s
ubuntu-22.04 3.2	34s
ubuntu-22.04 3.3	31s
ubuntu-22.04 head	23s
ubuntu-22.04 jruby	33s
ubuntu-22.04 jruby-head	30s
macos-13 2.7	2m 14s
macos-13 3.0	45s
macos-13 3.1	40s
macos-13 3.2	34s
macos-13 3.3	40s
macos-13 head	31s
macos-13 jruby	36s
macos-13 jruby-head	42s
macos-14 2.7	1m 27s
macos-14 3.0	23s
macos-14 3.1	37s
macos-14 3.2	22s
macos-14 3.3	23s
macos-14 head	21s
macos-14 jruby	25s
macos-14 jruby-head	33s
windows-latest 2.7	2m 23s
windows-latest 3.0	1m 38s
windows-latest 3.1	1m 59s
windows-latest 3.2	2m 10s
windows-latest 3.3	3m 8s
windows-latest head	1m 10s
windows-latest mswin	48s
ubuntu-latest jruby-9.4	29s
macos-latest truffleruby-head	38s
ubuntu-latest truffleruby-head	1m 3s
	31m 33s

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