change unreachable{} to @unreachable()

instead of a container init expression, it's a builtin
function call.

std/hash_map.zig

@@ -55,7 +55,7 @@ pub struct SmallHashMap(K: type, V: type, hash: fn(key: K)->u32, eql: fn(a: K, b
                     return entry;
                 }
             }
-            unreachable{} // no next item
+            @unreachable() // no next item
         }
     }
     
@@ -137,9 +137,9 @@ pub struct SmallHashMap(K: type, V: type, hash: fn(key: K)->u32, eql: fn(a: K, b
                 entry.distance_from_start_index -= 1;
                 entry = next_entry;
             }
-            unreachable{} // shifting everything in the table
+            @unreachable() // shifting everything in the table
         }}
-        unreachable{} // key not found
+        @unreachable() // key not found
     }
 
     pub fn entryIterator(hm: &Self) -> Iterator {
@@ -210,7 +210,7 @@ pub struct SmallHashMap(K: type, V: type, hash: fn(key: K)->u32, eql: fn(a: K, b
             };
             return;
         }
-        unreachable{} // put into a full map
+        @unreachable() // put into a full map
     }
 
     fn internalGet(hm: &Self, key: K) -> ?&Entry {