Basic backward slicing tests

[rolyp]

Test cases for forward and backward slicing, starting with primitive operations, the infrastructure for which is quite complex.

@min-nguyen suggested using the prettyprinter for tests, which will make life simpler. For now, have added a bracketsIf prettyprinter which wraps a Doc in square brackets iff an annotation is true, and used it initially just for integers.

Basic backward slicing tests:

• [x] dependsBoth and dependsZero primitives
• [x] array projection
  • [x] get prettyprinting to level where useful for array project test
• [x] array dims
  • [x] pair prettyprinting
• [x] backward slicing a primitive should require slicing criterion to be related to original output
  • [x] new expand consolidation for arithmetic primitives
  • [x] can I make it work for array lookup too?
  • [x] divide test to verify bwd slicing primitive with hole
    • [x] hole-expand result slice
• [x] nth test
• examples for paper – see #446

[rolyp]

@min-nguyen Extracted from #284. Now that we have a more stable base, let’s try to move forward with smaller tasks.

[rolyp]

@min-nguyen For the map example, I suggest just mapping the identity function to a 3-element list to start with, and slicing out the middle element of the input.

[rolyp]

@min-nguyen Let’s leave this for now and come back to it when we start working on examples for the paper.