enum_map macro can cause UB when `Enum` trait is incorrectly implemented

Summary

Affected versions of this crate did not properly check the length of an enum when using `enum_map!` macro, trusting user-provided length.

When the `LENGTH` in the `Enum` trait does not match the array length in the `EnumArray` trait, this can result in the initialization of the enum map with uninitialized types, which in turn can allow an attacker to execute arbitrary code.

This problem can only occur with a manual implementation of the Enum trait, it will never occur for enums that use `#[derive(Enum)]`.

Example code that triggers this vulnerability looks like this:

```rust enum E { A, B, C, }

impl Enum for E { const LENGTH: usize = 2;

fn from_usize(value: usize) -> E { match value { 0 => E::A, 1 => E::B, 2 => E::C, _ => unimplemented!(), } }

fn into_usize(self) -> usize { self as usize } }

impl<V> EnumArray<V> for E { type Array = [V; 3]; }

let _map: EnumMap<E, String> = enum_map! { _ => "Hello, world!".into() }; ```

The flaw was corrected in commit [b824e23](https://github.com/xfix/enum-map/commit/b824e232f2fb47837740070096ac253df8e80dfc) by putting `LENGTH` property on sealed trait for macro to read.