[features] Share more code, do less work #3266

Job	Run time
Clippy lints	1m 37s
fontc tests	1m 16s
Rustfmt	1m 15s
cargo check no std	1m 32s
resources/scripts/ots_test.sh	1m 10s
cargo check wasm	1m 18s
fontbe tests	54s
cargo test fea-rs	1m 7s
Check if .glyphs files have unnecessary elements	2s
Build GS and check the result	3m 7s
tests other than fontbe,fontc	2m 12s
	15m 30s

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