Move lexer to use regex instead of hand-rolled lexing

Signed-off-by: Alek Ratzloff <alekratz@gmail.com>

src/syn/span.rs

@@ -36,6 +36,16 @@ impl Pos {
         }
     }
 
+    pub fn adv_char(&mut self, c: char) {
+        *self = self.next_char(c);
+    }
+
+    pub fn adv_str(&mut self, s: &str) {
+        for c in s.chars() {
+            self.adv_char(c);
+        }
+    }
+
     pub fn min(self, other: Self) -> Self {
         if self.byte < other.byte {
             self