heapless [breaking-change] use hash32 v0.3.0

this release of hash32 has the advantage that 32-bit hashers can be used to hash types that implement the core::hash::Hash trait removing the need for the hash32::Hash implementations in this crate and the uses of the #[derive(Hash32)] macro (which did not support enums) in dependent crates

with this change the following code works

// NOTE no derive(Hash32)
struct Int(i32);

let mut x = FnvIndexSet::<_, 4>::default();
let _ = x.insert(Int(0));

this change is technically a breaking change because the following code is no longer accepted

// assume this type comes from a dependency
// NOTE no derive(Hash)
struct Int(i32);

let mut x = FnvIndexSet::<_, 4>::default();
let _ = x.insert(Int(0)); // error: does not implement Hash