read: implement Default for DwarfSections and DwarfPackageSections #522

Job	Run time
bench	1m 2s
Build fuzz targets	1m 7s
build (ubuntu-latest, stable)	53s
coverage	1m 5s
build (ubuntu-latest, beta)	47s
cross (i686-unknown-linux-gnu)	42s
build (ubuntu-latest, nightly)	43s
cross (powerpc64-unknown-linux-gnu)	55s
doc	19s
build (stable, macOS-latest)	53s
features	2m 56s
msrv-read	2m 20s
rustfmt	9s
msrv	2m 23s
Run `debug_abbrev` fuzz target	5m 10s
Run `debug_aranges` fuzz target	5m 6s
Run `debug_info` fuzz target	5m 11s
Run `debug_line` fuzz target	5m 6s
Run `eh_frame` fuzz target	5m 6s
Run `eh_frame_hdr` fuzz target	5m 7s
	47m 0s

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