Add EnumCount derive

[Boscop]

I wrote this proc-macro because I needed it (for mapping Enum cases from different enums to Vec regions, making the Vec len equal to the combined number of all cases) and I think it would make sense to include it in this crate, because it already contains many useful derives for enums.
https://github.com/Boscop/enum-count/blob/master/src/lib.rs
https://github.com/Boscop/enum-count-derive/blob/master/src/lib.rs

It allows stuff like this:

#[derive(EnumCount)] enum Aa { }

#[derive(EnumCount)]
enum Bb { B0 = AA_COUNT as _ }

#[repr(u8)]
#[derive(EnumCount)]
enum Cc { C0 = AA_COUNT as _, C1 = BB_COUNT as _ }

#[repr(usize)]
#[derive(EnumCount)]
enum Dd { D0 = AA_COUNT, D1 = BB_COUNT, D2 = CC_COUNT }

fn main() {
	println!("{} {} {} {}", Aa::count(), BB_COUNT, Cc::count(), DD_COUNT);
}

(The constant is necessary for it to be used in other enums and static array lens etc.)

[Peternator7]

This makes sense to me. Would you be willing to make the changes and submit a PR?

Once rust-lang/rfcs#911 is stablized, we can make count() const as well.

[Dushistov]

I also need this feature too. I will try to make PR.