utf16_chars incorrect for even-numbered non-BMP planes

[SimonSapin]

The code uses c |= 0x1_0000_u32 where it should use c += 0x1_0000_u32.

Test case:

use std::str;

fn main() {
    let a = "\U00020000";
    let b = a.to_utf16();
    let c = str::from_utf16(b);
    println!("{:?}", a);
    println!("{:?}", b);
    println!("{:?}", c)
}

Output:

"\U00020000"
~[55360u16, 56320u16]
~"\U00010000"

Expected output:

"\U00020000"
~[55360u16, 56320u16]
~"\U00020000"