CtString should lower to CtString from comptime to runtime

[asterite]

Once #10678 gets merged, all values with a type T will lower to a type T from comptime to runtime except CtString. To avoid this exception, CtString should be lowered into a CtString. Users should still have a way to convert a CtString into a string, the same way they can convert a fmtstr into a string.

Lowering a CtString

Given this code:

use std::append::Append;

fn main() {
    let ct_string = comptime {
        let mut ct_string = CtString::new();
        ct_string = ct_string.append("Hello");
        ct_string = ct_string.append(" world");
        ct_string
    };
}

It can be lowered like this:

fn main() {
    let ct_string = comptime { std::append::Append::append(CtString::new(), "Hello world") };
}

That is, we can always lower it with an expression std::append::Append::append(CtString::new(), <CtString contents>).

Turning a CtString into str

CtString already has a as_quoted_str method for this.