build

Recommend -Zshare-generics=false with opt-level=z #153

Job	Run time
no_std (ubuntu-latest, nightly)	14s
no_std (ubuntu-latest, stable)	27s
no_std-win (nightly)	58s
no_std (macos-latest, stable)	10s
no_std (macos-latest, nightly)	21s
no_std-win (stable)	1m 44s
no_main-win (stable)	42s
Default (ubuntu-latest, stable)	13s
Default (windows-latest, nightly)	39s
Default (macos-latest, stable)	11s
build-std (build_std)	46s
Default (ubuntu-latest, nightly)	15s
no_main-win (nightly)	32s
Default (windows-latest, stable)	34s
Default (macos-latest, nightly)	19s
build-std (no_main/nix)	45s
	8m 50s

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