Refactor enc_ciphertext to return reference instead of copy #64

Job	Run time
Build target wasm32-wasi	2m 19s
Test on ubuntu-latest	2m 14s
Latest build on ubuntu-latest	16s
Test on windows-latest	3m 41s
Latest build on windows-latest	58s
Test on macOS-latest	2m 10s
Bitrot check	2m 9s
Latest build on macOS-latest	10s
Clippy (MSRV)	2m 11s
Intra-doc links	2m 17s
Rustfmt	13s
	18m 38s

Provide feedback