Avoid String allocations in BLT parser

src/parser/blt.rs

@@ -191,10 +191,23 @@ impl<N: Number, I: Iterator<Item=char>> BLTParser<N, I> {
 	
 	/// Parse an integer into a [usize]
 	fn usize(&mut self) -> Result<usize, ParseError> {
-		return Ok(self.integer()?.parse().expect("Invalid usize"));
+		// return Ok(self.integer()?.parse().expect("Invalid usize"));
+		// Use a separate implementation to avoid allocating String
+		
+		let mut result = self.digit_nonzero()?.to_digit(10).unwrap() as usize;
+		loop {
+			match self.digit() {
+				Err(_) => { break; }
+				Ok(d) => {
+					result = result.checked_mul(10).expect("Integer overflows usize");
+					result = result.checked_add(d.to_digit(10).unwrap() as usize).expect("Integer overflows usize");
+				}
+			}
+		}
+		return Ok(result);
 	}
 	
-	/// Parse an integer as a [String]
+	/*/// Parse an integer as a [String]
 	fn integer(&mut self) -> Result<String, ParseError> {
 		let mut result = String::from(self.digit_nonzero()?);
 		loop {
@@ -204,7 +217,7 @@ impl<N: Number, I: Iterator<Item=char>> BLTParser<N, I> {
 			}
 		}
 		return Ok(result);
-	}
+	}*/
 	
 	/// Parse a number as an instance of N
 	fn ballot_value(&mut self) -> Result<(), ParseError> {