Switch to i16 for backet depth count.

This is having a measurable performance increase. 32k bracket depth should be enough for any non-malicious document.

src/parser/org_source.rs

@@ -17,10 +17,10 @@ pub struct OrgSource<'s> {
     start: usize,
     end: usize, // exclusive
     start_of_line: usize,
+    bracket_depth: i16,     // []
+    brace_depth: i16,       // {}
+    parenthesis_depth: i16, // ()
     preceding_character: Option<char>,
-    bracket_depth: i32,     // []
-    brace_depth: i32,       // {}
-    parenthesis_depth: i32, // ()
 }
 
 impl<'s> std::fmt::Debug for OrgSource<'s> {
@@ -71,15 +71,15 @@ impl<'s> OrgSource<'s> {
         self.slice(..(other.start - self.start))
     }
 
-    pub fn get_bracket_depth(&self) -> i32 {
+    pub fn get_bracket_depth(&self) -> i16 {
         self.bracket_depth
     }
 
-    pub fn get_brace_depth(&self) -> i32 {
+    pub fn get_brace_depth(&self) -> i16 {
         self.brace_depth
     }
 
-    pub fn get_parenthesis_depth(&self) -> i32 {
+    pub fn get_parenthesis_depth(&self) -> i16 {
         self.parenthesis_depth
     }
 }