The typechecker should forbid using type names as identifiers

It is currently possible to write
```perk
fun main() : void {
    let void := 10;
}
```
This compiles to C, but obviously the resulting C program is not valid. The typechecker should avoid these cases.

Care should go into checking whether more complex cases such as
```perk
fun main() : void {
    let A := 10;
}

model A {
    let a :+ 0
}
```
should also be forbidden or not.