roc: `Num.intCast` enables segfaults by blessing negative `Nat`s

This is the root cause of https://github.com/rtfeldman/roc/issues/3377.

Here is an example of Num.intCast enabling a negative list capacity, which the REPL accepts:

» List.withCapacity -100

── TYPE MISMATCH ───────────────────────────────────────────────────────────────

The 1st argument to withCapacity is not what I expect:

4│      List.withCapacity -100
                          ^^^^

This argument is a number of type:

    I8, I16, I32, I64, I128, F32, F64, or Dec

But withCapacity needs the 1st argument to be:

    Nat


» List.withCapacity (Num.intCast -100)

[] : List a

» List.withCapacity (Num.intCast -100) |> List.sum |> Num.toStr

"0" : Str

» 

Here is an example of a compiled app rejecting the same negative list capacity:

[jan@framey roc]$ cat withoutIntCast.roc 
app "test"
    packages { pf: "examples/hello-world/platform/main.roc" }
    imports []
    provides [main] to pf

main = List.withCapacity -100 |> List.sum |> Num.toStr
[jan@framey roc]$ 
[jan@framey roc]$ 
[jan@framey roc]$ cargo run withoutIntCast.roc 
    Finished dev [unoptimized + debuginfo] target(s) in 0.40s
     Running `target/debug/roc withoutIntCast.roc`
🔨 Rebuilding host...

── TYPE MISMATCH ────────────────────────────────────────── withoutIntCast.roc ─

The 1st argument to withCapacity is not what I expect:

6│  main = List.withCapacity -100 |> List.sum |> Num.toStr
                             ^^^^

This argument is a number of type:

    I8, I16, I32, I64, I128, F32, F64, or Dec

But withCapacity needs the 1st argument to be:

    Nat

────────────────────────────────────────────────────────────────────────────────

1 error and 0 warnings found in 582 ms.

You can run the program anyway with: roc run withoutIntCast.roc
[jan@framey roc]$ 
[jan@framey roc]$ 
[jan@framey roc]$ cat withIntCast.roc 
app "test"
    packages { pf: "examples/hello-world/platform/main.roc" }
    imports []
    provides [main] to pf

main = List.withCapacity (Num.intCast -100) |> List.sum |> Num.toStr
[jan@framey roc]$ 
[jan@framey roc]$ 
[jan@framey roc]$ cargo run withIntCast.roc 
    Finished dev [unoptimized + debuginfo] target(s) in 0.40s
     Running `target/debug/roc withIntCast.roc`
🔨 Rebuilding host...
Segmentation fault (core dumped)
[jan@framey roc]$