Deny unreachable-pub

It's very useful when `pub` is equivalent to "this is crate's public
API", let's enforce this!

Ideally, we should enforce it for local `cargo test`, and only during
CI, but that needs https://github.com/rust-lang/cargo/issues/5034.

crates/syntax/src/parsing/text_token_source.rs

@@ -65,7 +65,7 @@ fn mk_token(pos: usize, token_offset_pairs: &[(Token, TextSize)]) -> parser::Tok
 
 impl<'t> TextTokenSource<'t> {
     /// Generate input from tokens(expect comment and whitespace).
-    pub fn new(text: &'t str, raw_tokens: &'t [Token]) -> TextTokenSource<'t> {
+    pub(crate) fn new(text: &'t str, raw_tokens: &'t [Token]) -> TextTokenSource<'t> {
         let token_offset_pairs: Vec<_> = raw_tokens
             .iter()
             .filter_map({