fix some lint warnings

src/debray_allocator.rs

@@ -106,7 +106,7 @@ impl BranchStack {
         let branch_num = self
             .last()
             .map(|occurrences| occurrences.current_branch_num.clone())
-            .unwrap_or_else(|| BranchNumber::default());
+            .unwrap_or_default();
 
         BranchDesignator { branch_num }
     }

src/parser/ast.rs

@@ -4,7 +4,6 @@
 use crate::arena::*;
 use crate::atom_table::*;
 use crate::offset_table::*;
-use crate::parser::char_reader::*;
 use crate::types::HeapCellValueTag;
 
 use std::cell::{Cell, Ref, RefCell, RefMut};