make Db Send?

src/lib.rs

@@ -10,6 +10,7 @@ use std::{
     sync::Arc,
 };
 
+#[macro_export]
 macro_rules! field_ptr {
     ($base_ptr: expr, $type: ty, $field: ident) => {{
         let base: FilePointer<$type> = $base_ptr;
@@ -333,6 +334,8 @@ pub struct Db<R> {
     _phantom: PhantomData<R>,
 }
 
+unsafe impl<R> Send for Db<R> where R: Send {} // TODO
+
 struct SnapshotAndFreeList<R> {
     snapshot: Arc<Snapshot<R>>,
     to_free: Vec<FileRange>,