Rollup merge of rust-lang#66015 - popzxc:refactor-librustc_parser, r=matklad

librustc_lexer: Refactor the module

This PR introduces a refactoring of the `librustc_lexer` in order to improve readability.

All the changes performed are only cosmetic and do not introduce any changes the lexer logic or performance.

Newly introduced modules `literal`, `token` and `utils` are just copy-pasted from the `lib.rs` and do not contain even cosmetic changes (I decided to do so so it'll be easier to review changes looking only on diff).

r? @petrochenkov

cc @Centril @matklad

Author: Centril

src/librustc_lexer/src/cursor.rs

@@ -41,10 +41,20 @@ impl<'a> Cursor<'a> {
     /// If requested position doesn't exist, `EOF_CHAR` is returned.
     /// However, getting `EOF_CHAR` doesn't always mean actual end of file,
     /// it should be checked with `is_eof` method.
-    pub(crate) fn nth_char(&self, n: usize) -> char {
+    fn nth_char(&self, n: usize) -> char {
         self.chars().nth(n).unwrap_or(EOF_CHAR)
     }
 
+    /// Peeks the next symbol from the input stream without consuming it.
+    pub(crate) fn first(&self) -> char {
+        self.nth_char(0)
+    }
+
+    /// Peeks the second symbol from the input stream without consuming it.
+    pub(crate) fn second(&self) -> char {
+        self.nth_char(1)
+    }
+
     /// Checks if there is nothing more to consume.
     pub(crate) fn is_eof(&self) -> bool {
         self.chars.as_str().is_empty()