Auto merge of #222 - SuperFluffy:serde_0.8, r=cuviper
Use serde 0.8
I updated `bigint`, `complex`, and `rational` to use `serde 0.8`, and also fixed deserialization and the `serde` feature as such in the `rational` crate (didn't add any tests, but it compiles now).
Similar to https://github.com/rust-num/num/pull/196 for `num/complex`, “`use serde;`” needed to be removed in `num/rational`.