Asserts the maximum value that can be returned from Vec::len

[EFanZh]

Currently, casting Vec<i32> to Vec<u32> takes O(1) time:

// See <https://godbolt.org/z/hxq3hnYKG> for assembly output.
pub fn cast(vec: Vec<i32>) -> Vec<u32> {
    vec.into_iter().map(|e| e as _).collect()
}

But the generated assembly is not the same as the identity function, which prevents us from casting Vec<Vec<i32>> to Vec<Vec<u32>> within O(1) time:

// See <https://godbolt.org/z/7n48bxd9f> for assembly output.
pub fn cast(vec: Vec<Vec<i32>>) -> Vec<Vec<u32>> {
    vec.into_iter()
        .map(|e| e.into_iter().map(|e| e as _).collect())
        .collect()
}

This change tries to fix the problem. You can see the comparison here: https://godbolt.org/z/jdManrKvx.