Mutability for Immutable Composite Types

[IGI-111]

CS-FSSA-002

In Sway, mutability is specified upon declaration. In case a variable is defined as immutable, its content
cannot be changed. However, the mutability check is not recursive on composite types. This means we
can create a mutable reference to a field of an immutable variable like the following example:

let t : (u64, u64) = (17, 42);
let p : &mut u64 = &mut t.0;
*p = 18;