Implement implicit returns

[pzipper]

Currently, if a function doesn't return, the compiler panics due to Cranelift's verification pass.

Instead, the compiler should implicitly return from each function. If the function should return a value, it should return uninitialized memory instead.

For basic types such as integers, 0 shall be returned as a default, similar to C.

This will eventually be replaced with a check that doesn't allow a function to return implicitly if it returns a value, and a function would have to explicitly return uninit instead:

const Main = func main(): i32 {
    // ...
    
    return uninit;
}

Equivalent to the following C code:

int main() {
    // implicit return
}