Make fbuffer_inc_capa easier to inline #96

Job	Run time
ruby-versions / Generate Ruby versions	3s
ubuntu-22.04 2.7	1m 9s
ubuntu-22.04 3.0	30s
ubuntu-22.04 3.1	35s
ubuntu-22.04 3.2	37s
ubuntu-22.04 3.3	32s
ubuntu-22.04 head	22s
ubuntu-22.04 jruby	1m 12s
ubuntu-22.04 jruby-head	1m 20s
macos-13 2.7	2m 42s
macos-13 3.0	37s
macos-13 3.1	35s
macos-13 3.2	33s
macos-13 3.3	34s
macos-13 head	42s
macos-13 jruby	2m 12s
macos-13 jruby-head	1m 59s
macos-14 2.7	1m 2s
macos-14 3.0	27s
macos-14 3.1	38s
macos-14 3.2	21s
macos-14 3.3	23s
macos-14 head	25s
macos-14 jruby	57s
macos-14 jruby-head	42s
windows-latest 2.7	5m 24s
windows-latest 3.0	2m 39s
windows-latest 3.1	2m 27s
windows-latest 3.2	2m 7s
windows-latest 3.3	3m 5s
windows-latest head	1m 36s
windows-latest mswin	1m 45s
ubuntu-latest jruby-9.4	1m 11s
macos-latest truffleruby-head	1m 7s
ubuntu-latest truffleruby-head	1m 2s
	43m 32s

Provide feedback