clippy: sort out `large_enum_variant` lint

[eightfilms]

Context: https://github.com/zksecurity/noname/blob/61334793c0035cf90841e76e2d100f8e547150fd/src/backends/mod.rs#L34-L35

This might be helpful: https://rust-lang.github.io/rust-clippy/master/index.html#/large_enum_variant