std::chars::Char::escape_unicode generates old-style \u and \U escapes, breaking syntax::print

[emk]

CC @pnkfelix, #19739.

Thank you for your work on the Unicode escape migration! This is probably already on your to-do list, but I thought I'd submit a test-case just to be safe.

The following test case:

fn main() {
    let mut escaped = String::from_str("");
    for c in '\u{10401}'.escape_unicode() {
        escaped.push(c);
    }
    assert_eq!("\\u{10401}", escaped);
}

…gives us:

assertion failed: `(left == right) && (right == left)` (left: `\u{10401}`, right: `\U00010401`)

Among other things, this causes issues for syntax::print and rust-peg, as described in kevinmehall/rust-peg#34.

[pnkfelix]

reopening and tagging as E-needstest.