Move menu item to admin/build/block/instances

The current menu location seems like at an unnecessarily high-level position. It'd make sense to move it under the main "Blocks" administration area, and make it a local task to match the "List" and "Add" functions of blocks. Since block instances are a relatively complex concept, it's also nice to move that section to a place that's deeper in the site configuration than other more basic configurations (like Contact form, Modules, Themes, etc. which are all quickly understood).