feat: Allow numeric generics to be referenced and add map

[jfecher]

Related issue(s)

Resolves #972

Description

Summary of changes

Allows numeric generics to be referenced like normal values. This resolves a common stumbling block for users when writing a function like the following:

fn foo<N>(array: [Field; N]) {
    for i in 0 .. N {
        ....
    }
}

Previously, we would error that there is no N in scope (since N is a type and not a value). With this PR, this code works as expected.

Notably, this PR now allows us to write the array map function and add it to our stdlib. This was not possible previously due to the syntax limitations placed on array-length expressions which prevented us from initializing an array using let mut ret = [0; array.len()];. This has been reworked slightly to reuse the same array length evaluation mechanism types use, which notably now allows us to use numeric generic type variables.

Dependency additions / changes

Test additions / changes

Added a test for the new array map function in higher_order_functions.

Checklist

• I have tested the changes locally.
• I have formatted the changes with Prettier and/or cargo fmt with default settings.
• I have linked this PR to the issue(s) that it resolves.
• I have reviewed the changes on GitHub, line by line.
• I have ensured all changes are covered in the description.

Documentation needs

• This PR requires documentation updates when merged.

We should document numeric generics are now useable like any other Field variable. Additionally, the new map method for arrays will need to be documented.