sign: make to-be-signed function publicly for async signing #265

Job	Run time
examples (stable)	12s
examples (beta)	18s
doc	17s
clippy	11s
examples (nightly-2023-04-01)	21s
build (stable)	16s
build (beta)	20s
formatting	55s
build (nightly-2023-04-01)	22s
Rust 1.58.0 MSRV	1m 57s
Build for a no_std target (stable)	12s
Rust 1.59.0 MSRV	1m 56s
test (stable)	36s
Build for a no_std target (beta)	18s
test (beta)	22s
Build for a no_std target (nightly-2023-04-01)	19s
coverage	3m 38s
test (nightly-2023-04-01)	25s
deny	4m 10s
udeps	4m 17s
	21m 22s

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