build

Switch to new `panic=immediate-abort` flag #164

Job	Run time
Default (macos-latest, stable)	23s
Default (macos-latest, nightly)	16s
Default (windows-latest, stable)	37s
Default (ubuntu-latest, stable)	15s
no_std-win (stable)	31s
no_std (macos-latest, stable)	16s
Default (ubuntu-latest, nightly)	16s
no_std (ubuntu-latest, stable)	14s
no_main-win (nightly)	42s
Default (windows-latest, nightly)	56s
no_std-win (nightly)	52s
build-std (no_main/nix)	52s
no_std (ubuntu-latest, nightly)	14s
no_main-win (stable)	41s
no_std (macos-latest, nightly)	24s
build-std (build_std)	51s
	8m 20s

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