Add more minor improvements and tests with QuickCheck

[d3adb5]

Built on top of #3, this PR adds further minor improvements and tests that use QuickCheck for property testing.

Summary of changes since #3:

• Core.Core:
  • 87d59ed adab2a0 Tests were included for the conversion to/from String for FileMode.
  • 802ef1a Instances of Read and Show for FileMode replaced the parse/unparse functions.
• Core.Object:
  • fc3059a The newtype keyword is now used for Tree and Blob for added strictness.
  • 42a8afc The looseObjectExists and hashPath functions were refactored for the sake of brevity.
  • 7886f94 A property test was added for storeObject, using a new instance of Arbitrary for Blob.
  • 3950c65 With the help of the filepath and extra packages, storeObject was rewritten.
  • 92be468 Instance of Read and Show for ObjectType replaced the parse/unparse functions.
  • 55cc3cd Code for the Commit, Tree, and Blob objects was split into separate files. Kind of WIP.
  • Haddock additions: 4aa9e71 ad63c56
• Util.Util:
  • 5a35064 zipMap now uses intersectionWith, from Data.Map.
  • 366dfea listDirectoryRecursive was rewritten using fewer lines, with a change in the order of the output list.
  • 55cc3cd compress and decompress were moved to the module.
  • Unit testing: 2ed6fe6 e081483 bf83c86 385ca6f 811166c
  • Haddock additions: 21a13cd e84c817 a451c3a

Until #3 is merged, this pull request will remain as a draft.

[d3adb5]

This should be ready for a review whenever you're available, @lucasoshiro. I would've liked to put more time into it, but life got busier. Don't want to hold you back in case you're not devoting time to the project because of this PR. :P

[d3adb5]

@lucasoshiro Humble reminder to review this PR... :smiling_face_with_tear: