Fix semantics of empty blocks

[smarr]

This PR does make sure that empty blocks evaluate to nil.
On the way of getting there, it does a bit more cleanup:

• make method generation context less mutable
• add Parser.toString()
• replace Vector in MethodGenerationContext with an ArrayList (don't need the synchronized)

@sophie-kaleba could you take a look at this one and review/approach it?
I'd assume grSOM will need this update, too.

This addresses SOM-st/SOM#36 and SOM-st/SOM#54.

[sophie-kaleba]

Looks fine to me. I'll add it to the pile of things I have to update in grSOM!