Use associated constant for EnumCount

[Boscop]

Use an associated constant for EnumCount:

pub trait EnumCount {
    const COUNT: usize;
}

This would replace both the count() method

strum/strum_macros/src/macros/enum_count.rs

Line 23 in a20ee16

fn count() -> usize {

and the <enum-name>_COUNT constant

strum/strum_macros/src/macros/enum_count.rs

Line 29 in a20ee16

pub const #const_name: usize = #n;

and it works on stable since 1.20.

(There is no reason why this needs to be a method (not even a const-fn method), and it's unelegant to have both a method and a non-associated constant.)

An associated constant is the right way to express this.

Related discussion: #58 #15 (comment)

---

Btw, this was motivated by my real-world use case that is currently not possible with EnumCount, I need to define the following:

pub trait Param: From<usize> + Into<usize> + Serialize + DeserializeOwned {
	const PARAM_COUNT: usize;
}

impl<T: From<usize> + Into<usize> + EnumCount> Param for T
where
	[T; <T as EnumCount>::count()]: Serialize + DeserializeOwned,
{
	const PARAM_COUNT: usize = T::count();
}

It's not possible because count() is not a constant.