pset: blind: return ephemeral private key #577

Job	Run time
Prepare	4s
Test - stable toolchain	3m 1s
Docs - stable toolchain (recent)	36s
Check WASM	33s
Lint - nightly toolchain (recent)	47s
Test - nightly toolchain	2m 48s
Docs - nightly toolchain (recent)	35s
Test - MSRV	9m 41s
	18m 5s

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