move hashable_row to dbsp.rs

There will be a new type for joins, so it makes less sense to have
a separate file just for it. dbsp.rs is good.

core/incremental/mod.rs

@@ -2,7 +2,6 @@ pub mod compiler;
 pub mod cursor;
 pub mod dbsp;
 pub mod expr_compiler;
-pub mod hashable_row;
 pub mod operator;
 pub mod persistence;
 pub mod view;