Refactor `signing` module into `privval` #1157

Job	Run time
Check	1m 44s
Test Suite (stable)	1m 17s
Clippy	2m 1s
Test Suite (1.72.0)	1m 16s
Build (stable)	8m 53s
Build (1.72.0)	7m 2s
Rustfmt	17s
Validate against test harness	1m 1s
	23m 31s

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