Allow referencing numeric generics as values

[jfecher]

Problem

Currently, given a function like

fn foo<N>(array: [Field; N]) -> Field {
    ...
}

Users would not be able to reference N directly as a value within foo since N is considered to be a type.

Proposed solution

Add N to the value scope as well and allow it to be referenced. During monomorphisation the value will be substituted with the numeric constant required by the caller as normal.

Alternatives considered

No response

Additional context

No response