Casting of primitive types

[baszalmstra]

We want to add the functionality to be able to cast from primitive types to other primitives through the use of the as keyword.

This should world:

pub fn test(a:i32) -> i64 {
    a as i64
}

[Discussion] as performs a bitcast, which avoids any security checks. Is this desirable or should we make it more difficult to write a bitcast?

[Discussion] Should we only allow as conversion if they are guaranteed to succeed? If not, a compile error will occur.

[Discussion] Should we allow fallible conversions and panic! if they fail (e.g. due to wrapping)?