Split blocks.js into one file per class:
- block.js
  - value_block.js (ValueBlock)
    - dependent_input_block.js (DependentInputBlock)
      - side_input_block.js (SideInputBlock)
    - var_args_block.js (VarArgsBlock)
TODO: Add in goog.requires() statements.
Minor changes to blocks-*.js, including adding goog.requires()
statements and switching from explicit loops to forEach.
TODO: blocks-textutils.js, blocks-turtle.js, blocks-vector.js.