[stdlib] Add `unreachable` check instead of `constrained[False]`

[Benny-Nottonson]

As mentioned in #2547, there should realistically be an unreachable function, to indicate when a segment of code can never be reached.

For example

fn one_or_two[value: Int]() -> Int:
    if value == 1:
        return 1
    elif value == 2:
        return 2
    unreachable()

The above code demonstrates a great use case, where the alternative now would be:

fn one_or_two[value: Int]() -> Int:
    constrained[value == 1 or value == 2]()
    if value == 1:
        return 1
    return 2

This is problematic in my opinion as it creates two issues:

1. You are now checking value == 1 two separate times, the only way to avoid this is creating an unnecessary variable.
2. You are reducing readability and harming code structure by leading with the entire body of a conditional.

More information and further suggestions found at https://github.com/modularml/mojo/issues/2547

Signed-off-by: benny-nottonson [email protected]

[Brian-M-J]

contrained[value == 1 or value == 2]()

I assume you mean constrained?

[Benny-Nottonson]

contrained[value == 1 or value == 2]()

I assume you mean constrained?

my mistake, fixed