UnrestrictedSlots implementation

[bugadani]

Based on #11 provide a data structure that imposes less restrictions than the original Slots.

This PR rebases the original code to the current master, renames RawSlots to UnrestrictedSlots (still probably not the final name) and implements iterator traits for it.

• Documentation
• Code cleanup
• Name

[codecov-commenter]

Codecov Report

Merging #17 into master will increase coverage by 0.57%.
The diff coverage is 64.02%.

@@            Coverage Diff             @@
##           master      #17      +/-   ##
==========================================
+ Coverage   59.05%   59.62%   +0.57%     
==========================================
  Files           3        6       +3     
  Lines         254      322      +68     
  Branches       56       58       +2     
==========================================
+ Hits          150      192      +42     
- Misses         50       70      +20     
- Partials       54       60       +6     

Flag	Coverage Δ
#unittests	59.62% <64.02%> (+0.57%)	⬆️

Impacted Files	Coverage Δ
src/lib.rs	24.44% <ø> (-27.67%)	⬇️
src/private.rs	0.00% <ø> (ø)
tests/test.rs	73.78% <ø> (ø)
src/unrestricted.rs	47.88% <48.57%> (ø)
src/slots.rs	69.81% <69.81%> (ø)
src/iterator.rs	82.92% <82.92%> (ø)
... and 1 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 720c2d9...d99eda3. Read the comment docs.