Add typecast page to Docs

[alphak3y]

Currently there is no documented way to typecast from a u32 to a u64.

[mohammadfawaz]

So Sway has implicit casting for integers. So you could do

    let y:u32 = 5;
    let x:u64 = y;

Is this what you're referring to? The compiler will also warn you if there is a potential loss of precision (i.e. downcasting).

[mohammadfawaz]

@anton-trunov assigning this to you. Once we get rid of implicit casting and add into() for integers (like Rust), we should document that behaviour in the Sway book.

[anton-trunov]

We cannot have into() for Sway as of now, because the From trait has both directions:

pub trait From<T> {
    fn from(b: T) -> Self;
    fn into(self) -> T;
}

meaning that the types T and U in From<T> for U are isomorphic (see this issue: #3637).

So, currently the way to go would be let x : u64 = 5u32.as_u64().