gc: add `--expire-to` option #350

Job	Run time
config	3s
fuzz smoke test	36s
win+VS build	0s
win build	56s
sparse	50s
static-analysis	1m 3s
linux32 (i386/ubuntu:focal)	53s
linux-musl (alpine)	51s
pedantic (fedora)	52s
documentation	1m 3s
linux-asan-ubsan (ubuntu-latest)	1m 4s
linux-gcc (ubuntu-20.04)	1m 5s
linux-gcc-default (ubuntu-latest)	1m 3s
linux-leaks (ubuntu-latest)	1m 3s
win+VS test	0s
linux-reftable (ubuntu-latest)	1m 3s
linux-reftable-leaks (ubuntu-latest)	1m 2s
linux-sha256 (ubuntu-latest)	1m 4s
linux-TEST-vars (ubuntu-20.04)	1m 4s
osx-gcc (macos-13)	1m 10s
osx-clang (macos-13)	1m 2s
osx-reftable (macos-13)	1m 10s
win test	0s
	18m 57s

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