tr does not handle escape sequences correct

[SuperSandro2000]

autoconf relies on the following snippet to generate a CRLF character:

echo X | tr X '\015'

which is broken in uutils-coreutils and returns a 0:

uutils-tr

$ echo X | tr X '\015' | /bin/xxd
00000000: 300a                                     0.

gnus tr:

echo X | tr X '\015' | /bin/xxd
00000000: 0d0a                                     ..

[cole-h]

This seems to stem from here:

coreutils/src/uu/tr/src/expand.rs

Lines 52 to 56 in 2c09556

	'\\' if self.string.len() > 1 => {
	// yes---it's \ and it's not the last char in a string
	// we know that \ is 1 byte long so we can index into the string safely
	let c = self.string[1..].chars().next().unwrap();
	(Some(unescape_char(c)), 1 + c.len_utf8())

which only handles characters (e.g. it's only parsing the 0, where it should be parsing the whole thing as an octal). This could be solved by making unescape take &str and then checking if the &str starts with \ (and continue with that).

There would probably need to be "specialized" parsing for \u and \x, as well as octal (with support for those with and without leading octal numbers). POSIX tr doesn't specify \u and \x, so this is unnecessary.