[features] Share more code, do less work #3267

Job	Run time
Rustfmt	1m 15s
fontbe tests	55s
fontc tests	1m 13s
Clippy lints	1m 39s
cargo check no std	1m 33s
resources/scripts/ots_test.sh	1m 7s
cargo test fea-rs	1m 10s
tests other than fontbe,fontc	2m 11s
Build GS and check the result	3m 5s
cargo check wasm	1m 13s
Check if .glyphs files have unnecessary elements	4s
	15m 25s

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