feat: resolve generic length of slice inconsistency

[michaeljklein]

Description

Resolve generic N parameters applying inconsistently to arrays and slices.

Problem*

Resolves #4220

Summary*

In general, generic N parameters are expected to be able to be instantiated to any specific Type::Constant(n) or Type::NotConstant, however these two cases have slightly different semantics.

For example, the map definition uses the array length N from the type to pick the size of the new array, but this isn't known for slices, leading to a panic:

    // Apply a function to each element of an array, returning a new array
    // containing the mapped elements.
    pub fn map<U, Env>(self, f: fn[Env](T) -> U) -> [U; N] {
        let first_elem = f(self[0]);
        let mut ret = [first_elem; N];

        for i in 1 .. self.len() {
            ret[i] = f(self[i]);
        }

        ret
    }

In a new approach fn foo<T, N?>(x: [T; N]) would be needed to operate on slices as well. This would prevent N from being used as a numeric.

Additional Context

Documentation*

Check one:

• No documentation needed.
• Documentation included in this PR.
• [Exceptional Case] Documentation to be submitted in a separate PR.

PR Checklist*

• I have tested the changes locally.
• I have formatted the changes with Prettier and/or cargo fmt on default settings.

[Savio-Sou]

Also resolves #2540?

[michaeljklein]

replaced by #4504