Refactor enc_ciphertext to return reference instead of copy #68

Job	Run time
Build target wasm32-wasi	2m 14s
Bitrot check	2m 12s
Latest build on ubuntu-latest	14s
Latest build on windows-latest	48s
Clippy (MSRV)	2m 20s
Intra-doc links	2m 18s
Latest build on macOS-latest	17s
Rustfmt	10s
Test on ubuntu-latest	2m 21s
Test on windows-latest	4m 5s
Test on macOS-latest	2m 24s
	19m 23s

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