Change variable binding syntax

[PaulBone]

At the moment variable bindings allow a variable to be available after branching control flow if all paths either bind it or return early (not implemented):

if (...) { 
    x = ...
} else {
    x = ...
}
print!(x)   % We can use x here.

But it was pointed out to me that this is unintuitive, by more than one
person. In my own experience this is okay (it's how Mercury works, and no
one seems to worry about it / notice it there). But it seems I have some biases and instead we'll implement it as:

val x
if (...) { 
    x = ...
} else {
    x = ...
}
print!(x)   % We can use x here.

And track that work in this bug.

[brendanzab]

Yeah, this is pretty much how Rust does it!

let x; // `x` is uninitialized here
if ... {
    x = ...;
} else {
    x = ...;
}
println!("{}", x); // can only use `x` here because it was assigned in both arms

[PaulBone]

Yeah, this is pretty much how Rust does it!

I presume that:

let x; // `x` is uninitialized here
if ... {
    x = ...;
} else {
    not_x = ...;
}
println!("{}", x);

Is not legal.