Optimize scanning for in-buffer settings by scanning forward for possible keywords.

Previously we stepped through the document character by character which involved a lot of extra processing inside OrgSource. By scanning for possible keywords, we can skip many of the intermediate steps.

src/parser/org_source.rs

@@ -1,6 +1,7 @@
 use std::ops::RangeBounds;
 
 use nom::Compare;
+use nom::FindSubstring;
 use nom::InputIter;
 use nom::InputLength;
 use nom::InputTake;
@@ -77,6 +78,55 @@ impl<'s> OrgSource<'s> {
         self.slice(..(other.start - self.start))
     }
 
+    pub(crate) fn get_start_of_line(&self) -> OrgSource<'s> {
+        let skipped_text = self.text_since_line_break();
+        let mut bracket_depth = self.bracket_depth;
+        let mut brace_depth = self.brace_depth;
+        let mut parenthesis_depth = self.parenthesis_depth;
+        // Since we're going backwards, this does the opposite.
+        for byte in skipped_text.bytes() {
+            match byte {
+                b'\n' => {
+                    panic!("Should not hit a line break when only going back to the start of the line.");
+                }
+                b'[' => {
+                    bracket_depth -= 1;
+                }
+                b']' => {
+                    bracket_depth += 1;
+                }
+                b'{' => {
+                    brace_depth -= 1;
+                }
+                b'}' => {
+                    brace_depth += 1;
+                }
+                b'(' => {
+                    parenthesis_depth -= 1;
+                }
+                b')' => {
+                    parenthesis_depth += 1;
+                }
+                _ => {}
+            };
+        }
+
+        OrgSource {
+            full_source: self.full_source,
+            start: self.start_of_line,
+            end: self.end,
+            start_of_line: self.start_of_line,
+            preceding_character: if self.start_of_line > 0 {
+                Some('\n')
+            } else {
+                None
+            },
+            bracket_depth,
+            brace_depth,
+            parenthesis_depth,
+        }
+    }
+
     pub(crate) fn get_bracket_depth(&self) -> BracketDepth {
         self.bracket_depth
     }
@@ -310,6 +360,12 @@ impl<'s> InputTakeAtPosition for OrgSource<'s> {
     }
 }
 
+impl<'n, 's> FindSubstring<&'n str> for OrgSource<'s> {
+    fn find_substring(&self, substr: &'n str) -> Option<usize> {
+        Into::<&str>::into(self).find(substr)
+    }
+}
+
 pub(crate) fn convert_error<'a, I: Into<CustomError<&'a str>>>(
     err: nom::Err<I>,
 ) -> nom::Err<CustomError<&'a str>> {