Fix two bugs

bug 1: read-only or write-only resources were not
    added to the reverse dependcy lists,
    but accessed in find_runnable_task

bug 2: shred-derive did not build a hash value and
    did not use () as the default

shred-derive/src/lib.rs

@@ -34,20 +34,20 @@ fn impl_task_data(ast: &MacroInput) -> Tokens {
         impl<'a> ::shred::TaskData<'a> for #name<'a> {
             fn fetch(res: &'a ::shred::Resources) -> #name<'a> {
                 #name {
-                    #( #identifiers: unsafe { res.#methods(0) }, )*
+                    #( #identifiers: unsafe { res.#methods(()) }, )*
                 }
             }
 
             unsafe fn reads() -> Vec<::shred::ResourceId> {
                 use std::any::TypeId;
 
-                vec![ #( (TypeId::of::<#reads>(), 0) ),* ]
+                vec![ #( (TypeId::of::<#reads>(), 14695981039346656037) ),* ]
             }
 
             unsafe fn writes() -> Vec<::shred::ResourceId> {
                 use std::any::TypeId;
 
-                vec![ #( (TypeId::of::<#writes>(), 0) ),* ]
+                vec![ #( (TypeId::of::<#writes>(), 14695981039346656037) ),* ]
             }
         }
     }

src/dispatch.rs

@@ -26,9 +26,17 @@ impl Dependencies {
                 .entry(*read)
                 .or_insert(Vec::new())
                 .push(id);
+
+            self.rev_writes
+                .entry(*read)
+                .or_insert(Vec::new());
         }
 
         for write in &writes {
+            self.rev_reads
+                .entry(*write)
+                .or_insert(Vec::new());
+
             self.rev_writes
                 .entry(*write)
                 .or_insert(Vec::new())