Add basic String

[edg-l]

The String is a struct with a known layout, maybe mark it with a attribute telling its a lang item like in rust.

#[lang = "String"]
struct String {
    ptr: *mut u8,
    len: u64,
    cap: u64,
}


// The literal is of type String (converted by the compiler internally)
let x: String = "hello world!";