Add property tests for btree functions

[luc-tielen]

• Create new Spec module for testing btree functionality in LLVM.
• Specialize LLVM to a specific config: e.g. index on columns 1 and 3, ...
• Import the specialized, foreign LLVM instructions into the Spec module (only test at the level of what is exported via Functions datatype.
• Write property based tests for these functions (using hedgehog?).

[luc-tielen]

After some thinking, I found the following properties:

• purge: afterwards, empty = true / size = 0
• swap A B
  • swap 1x: B = A, A = B
  • swap 2x: A = A, B = B (identity)
• insert:
  • if empty tree: before empty = true, after empty = false
  • elem already in tree: no change
  • elem not yet in tree: size + 1
  • insert A + insert B = insert B + insert A
• insertRange: same as insert, size + max N (depends on how many elements already in tree)
• isEmpty: true for empty trees, false otherwise
• contains: true if elem is in tree, false otherwise

Other functions should be unit-tested / tested indirectly via "integration" tests:

• init(Empty)
• destroy
• begin
• end
• lowerBound
• upperBound