Refactor enc_ciphertext to return reference instead of copy #62

Job	Run time
Build target wasm32-wasi	2m 14s
Latest build on ubuntu-latest	20s
Latest build on windows-latest	53s
Test on ubuntu-latest	2m 14s
Bitrot check	2m 15s
Latest build on macOS-latest	14s
Test on windows-latest	3m 30s
Test on macOS-latest	1m 59s
Clippy (MSRV)	2m 17s
Intra-doc links	2m 18s
Rustfmt	12s
	18m 26s

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