Modularize blocks

[rachel-fenichel]

Issue for tracking conversion of blocks directory to goog.module.

Most of the files in blocks/ don't need to export anything, because they install blocks on the blocks object--that is, the side effects are the important thing (related to #5189).

Some files put properties on Blockly.Constants.SomeNamespace. These are generally not use outside of the file. They are on the constants namespace to keep them out of the global namespace. They can just become local and not exported, and it's possible that some of the leaf namespace on Blockly.Constants will also not be needed.