Add Nonnull, Nullable, and CheckReturnValue annotations

[sharwell]

💡 This pull request uses the annotations from com.google.code.findbugs:jsr305.

Fixes #48

[codecov]

Codecov Report

Merging #49 into master will increase coverage by 0.09%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##             master      #49      +/-   ##
============================================
+ Coverage     81.17%   81.26%   +0.09%     
- Complexity     1167     1168       +1     
============================================
  Files            40       40              
  Lines          6938     6934       -4     
  Branches        877      875       -2     
============================================
+ Hits           5632     5635       +3     
+ Misses         1011     1007       -4     
+ Partials        295      292       -3

Impacted Files	Coverage Δ
src/com/tvl/util/Requires.java	50% <ø> (ø)	✅
src/com/tvl/util/AbstractImmutableList.java	72.22% <ø> (ø)	✅
src/com/tvl/util/KeyValuePair.java	87.5% <ø> (ø)	✅
src/com/tvl/util/Immutables.java	94.11% <ø> (ø)	✅
src/com/tvl/util/ImmutableAtomic.java	89.68% <ø> (ø)	✅
src/com/tvl/util/EqualityComparators.java	18.75% <ø> (ø)	✅
src/com/tvl/util/Comparators.java	71.42% <ø> (ø)	✅
src/com/tvl/util/ImmutableTreeSet.java	87.23% <100%> (+0.04%)	✅
src/com/tvl/util/ImmutableLinkedQueue.java	96.77% <100%> (ø)	✅
src/com/tvl/util/SortedIntegerKeyNode.java	79.58% <100%> (+1.86%)	✅
... and 6 more

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