Proof of concept: Allow recycling UnwindContext allocation #419

Job	Run time
coverage	1m 14s
Build fuzz targets	1m 19s
bench	1m 26s
build (ubuntu-latest, stable)	59s
cross (i686-unknown-linux-gnu)	55s
build (ubuntu-latest, beta)	53s
doc	25s
features	3m 0s
cross (powerpc64-unknown-linux-gnu)	1m 3s
msrv-read	1m 51s
build (ubuntu-latest, nightly)	55s
rustfmt	12s
build (ubuntu-latest, 1.65.0)	2m 45s
build (stable, macOS-latest)	1m 27s
Run `debug_abbrev` fuzz target	5m 10s
Run `debug_aranges` fuzz target	5m 7s
Run `debug_info` fuzz target	5m 17s
Run `debug_line` fuzz target	5m 10s
Run `eh_frame` fuzz target	5m 7s
Run `eh_frame_hdr` fuzz target	5m 5s
	49m 20s

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