implement eat_until leveraging memchr in lexer

compiler/rustc_lexer/src/lib.rs

@@ -483,7 +483,7 @@ impl Cursor<'_> {
             _ => None,
         };
 
-        self.eat_while(|c| c != '\n');
+        self.eat_until(b'\n');
         LineComment { doc_style }
     }
 
@@ -888,7 +888,7 @@ impl Cursor<'_> {
         // Skip the string contents and on each '#' character met, check if this is
         // a raw string termination.
         loop {
-            self.eat_while(|c| c != '"');
+            self.eat_until(b'"');
 
             if self.is_eof() {
                 return Err(RawStrError::NoTerminator {