Made Self a keyword.

It is only allowed in paths now, where it will either work inside a `trait`
or `impl` item, or not resolve outside of it.

[breaking-change]

Closes #22137

src/libsyntax/parse/lexer/mod.rs

@@ -1122,7 +1122,7 @@ impl<'a> StringReader<'a> {
                 let keyword_checking_token =
                     &token::Ident(keyword_checking_ident, token::Plain);
                 let last_bpos = self.last_pos;
-                if keyword_checking_token.is_keyword(token::keywords::Self) {
+                if keyword_checking_token.is_keyword(token::keywords::SelfValue) {
                     self.err_span_(start,
                                    last_bpos,
                                    "invalid lifetime name: 'self \