Blocks language supports benchmark program

[asolove]

This PR contains changes to the Snap definitions of the Pyret language to enable us to construct the demo program we want to be able to run the benchmark program.

In previous testing, we found a few missing items:

• Introduce a new variable and assign it to a value: var = val

• Lookup for Row, StringDict and other map-like things: map[key]
  

• image-scatter-plot (turns out this already worked and Adam just misunderstood)

• load-table can define multiple column names as variables
  

• allow multiple arguments in function definitions and applications
  

[asolove]

@jpolitz I think this is everything needed to run our benchmark program. There are only changes to the blocks definitions. It would be helpful if we could merge this to horizon and deploy. Then we can test it out in an environment that can use context shared-gdrive.

[jpolitz]

I believe this is now live at https://pyret-horizon.herokuapp.com/blocks