Replace deprecated trim_left with trim_start

Author: theduke

src/common.rs

@@ -114,7 +114,7 @@ fn unquote_block_string<'a>(src: &'a str) -> Result<BorrowedStr<'a>, Error<Token
     debug_assert!(src.starts_with("\"\"\"") && src.ends_with("\"\"\""));
     let indent = src[3..src.len()-3].lines().skip(1)
         .filter_map(|line| {
-            let trimmed = line.trim_left().len();
+            let trimmed = line.trim_start().len();
             if trimmed > 0 {
                 Some(line.len() - trimmed)
             } else {