Add optional support for the arbitrary crate #671

Job	Run time
miri	11m 32s
minrust	1m 35s
loom	1m 46s
cross (i686-unknown-linux-gnu)	26s
nightly	3m 38s
cross (armv7-unknown-linux-gnueabihf)	27s
rustfmt	5s
stable (ubuntu-latest)	1m 24s
cross (powerpc-unknown-linux-gnu)	29s
tsan	2m 10s
cross (powerpc64-unknown-linux-gnu)	29s
stable (macos-latest)	1m 43s
cross (wasm32-unknown-unknown)	6s
stable (windows-latest)	3m 28s
Publish Documentation	0s
	29m 18s

Provide feedback