Cache symbol completion result #4043

Job	Run time
Debug compatibility test	5m 52s
lint	9s
ruby-versions / Generate Ruby versions	3s
rake test 2.7 (latest reline)	1m 33s
vterm-yamatanooroti 2.7 (latest reline)	45s
rake test 2.7	1m 33s
vterm-yamatanooroti 2.7	1m 1s
rake test 3.0 (latest reline)	1m 25s
vterm-yamatanooroti 3.0 (latest reline)	1m 5s
rake test 3.0	1m 32s
vterm-yamatanooroti 3.0	52s
rake test 3.1 (latest reline)	1m 34s
vterm-yamatanooroti 3.1 (latest reline)	30s
rake test 3.1	1m 34s
vterm-yamatanooroti 3.1	29s
rake test 3.2 (latest reline)	1m 29s
vterm-yamatanooroti 3.2 (latest reline)	32s
rake test 3.2	1m 44s
vterm-yamatanooroti 3.2	35s
rake test 3.3 (latest reline)	1m 26s
vterm-yamatanooroti 3.3 (latest reline)	28s
rake test 3.3	1m 43s
vterm-yamatanooroti 3.3	28s
rake test head (latest reline)	1m 51s
vterm-yamatanooroti head (latest reline)	44s
rake test head	2m 0s
vterm-yamatanooroti head	32s
rake test truffleruby-head (latest reline)	3m 4s
rake test truffleruby-head	2m 42s
	39m 15s

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