Use const generics for array impl

[Boscop]

min_const_generics is in stable now, so we can use it for the array impl:

serde-diff/src/implementation.rs

Line 17 in e461da4

impl<T: $crate::SerdeDiff + serde::Serialize + for<'a> serde::Deserialize<'a>> $crate::SerdeDiff for [T; $len] {

Other crates have also switched to const generics for array impls (such as arrayvec).