tokenizer: Source positions are always at char boundaries, so avoid overhead when slicing.

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Author: emilio

src/tokenizer.rs

@@ -274,6 +274,7 @@ impl<'a> Tokenizer<'a> {
 
     #[inline]
     pub fn position(&self) -> SourcePosition {
+        debug_assert!(self.input.is_char_boundary(self.position));
         SourcePosition(self.position)
     }
 
@@ -313,13 +314,15 @@ impl<'a> Tokenizer<'a> {
     }
 
     #[inline]
-    pub fn slice_from(&self, start_pos: SourcePosition) -> &'a str {
-        &self.input[start_pos.0..self.position]
+    pub(crate) fn slice_from(&self, start_pos: SourcePosition) -> &'a str {
+        self.slice(start_pos..self.position())
     }
 
     #[inline]
-    pub fn slice(&self, range: Range<SourcePosition>) -> &'a str {
-        &self.input[range.start.0..range.end.0]
+    pub(crate) fn slice(&self, range: Range<SourcePosition>) -> &'a str {
+        debug_assert!(self.input.is_char_boundary(range.start.0));
+        debug_assert!(self.input.is_char_boundary(range.end.0));
+        unsafe { self.input.get_unchecked(range.start.0..range.end.0) }
     }
 
     pub fn current_source_line(&self) -> &'a str {