Export Instruction type + constructor
Fix bug in instruction prism and add tests

* Add tests for the Haemu.Instruction module
* Factor out the Arbitrary (V.Vector a) instance to tests/Instances.hs
* Fix two bugs in the instruction prism:
   1. V.drop 2 instead of V.drop length (typo) to get datablocks
   2. There may be at most 15 datablocks in an instruction (More of a
      documentation bug)