pset: blind: return ephemeral private key #576

Job	Run time
Check WASM	32s
Docs - stable toolchain (recent)	41s
Test - stable toolchain	3m 4s
Prepare	5s
Test - MSRV	9m 35s
Lint - nightly toolchain (recent)	41s
Test - nightly toolchain	3m 9s
Docs - nightly toolchain (recent)	32s
	18m 19s

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