Simpler FSTRIPS ASTs

[gfrances]

The current FSTRIPS language is too involved and because of this we're having endless bugs over and over (well, because of this and because our automated tests are a bit rudimentary yet...).

There's some ongoing work to simplify this, I'll just open this issue as a big reminder and as a place to index all of the issues that are related to / would be strongly affected by this.

ATM these issues are, at least: #18, #51, #56, #57.