[readline] Ctrl-A and Ctrl-E results invalid position of cursor

[g1eng]

Cursor position after C-A or C-E is invalid.

For reproduction, set some prefix, input words and press C-A and C-E.

C-A results bit right position than valid place. and C-E results left side than valid one.

[g1eng]

testing with this implementation MANUALLY:

use std::env;
use std::io;
use ttyui::readline::Buffer;

fn main() -> io::Result<()> {
    let mut buf = Buffer::new();
    buf.set_prefix("oi kitarou> ".to_string());
    if let Some(x) = env::args().nth(1) {
        if x == "-d" || x == "--double" {
            buf.double_line_response = true;
        } else {
            panic!("unknown arguments");
        }
    }
    buf.read_line()?;
    println!("\n\n[output]\n\x1b[33m{}\x1b[0m", buf.to_string(),);
    Ok(())
}

[g1eng]

It seems to be a problem caused by simple misunderstanding on prefix length handling .

fixing end() and home()...

[g1eng]

fixed #4

[g1eng]

The problem is not fixed completely.
Try to place cursor on mid of the line and press C-A or C-E.
This is caused by invalid sequence calculation for cursor moving on home() and end(), which is not revealed without prefix string functionality.

[g1eng]

refixed #6