Bump proc-macro2 from 1.0.76 to 1.0.78 #55

Job	Run time
set-image	1s
clippy	1m 43s
bench-rust-nightly (bit-vec)	18m 26s
checks	1m 35s
bench-rust-nightly (bytes)	17m 1s
miri (bit-vec)	51m 49s
tests	2m 53s
bench-rust-nightly (generic-array)	17m 1s
miri (bytes)	2m 56s
bench-rust-nightly (derive)	17m 8s
miri (generic-array)	3m 2s
bench-rust-nightly (max-encoded-len)	16m 54s
miri (arbitrary)	2m 56s
build-linux-ubuntu-amd64	1m 24s
publish-dry-run	1m 49s
	2h 36m 38s

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