Add iter, keys, and values with Java tests

[thallada]

I was following part of you stream earlier today and decided to try my hand at porting some of the other Java tests from MapCheck.java.

The contains_key tests were pretty straightforward since you already implemented contains_key. Those are t4 and t7.

During the stream, you mentioned that NodeIter is a private Iterator which could be the basis for public iterators on keys, values, and entries. I've done that in a new file src/iter/iter.rs with three new structs Iter, Keys, and Values (similar to std::collections::HashMap). I added some of my own tests for the new iter(), keys(), and values() methods and ported ittest1, ittest2, and ittest3 from MapCheck.java.

To get the values out of the NodeIter, I had to add two more unsafe blocks.

I'm still a noob at concurrency and Rust so let me know if there's anything to improve here. I'm using this as a learning experience. Thanks for streaming!

[jonhoo]

I love this change, thank you! Could you rebase onto master so that you get the CI testing changes I just pushed?

[codecov]

Codecov Report

Merging #2 into master will increase coverage by 2.26%.
The diff coverage is 95.23%.

@@            Coverage Diff             @@
##           master       #2      +/-   ##
==========================================
+ Coverage    83.1%   85.37%   +2.26%     
==========================================
  Files           5        6       +1     
  Lines         586      670      +84     
==========================================
+ Hits          487      572      +85     
+ Misses         99       98       -1

Impacted Files	Coverage Δ
src/lib.rs	80.33% <100%> (+1.35%)	⬆️
src/iter/iter.rs	91.89% <91.89%> (ø)
tests/jdk/map_check.rs	98.43% <97.36%> (-1.57%)	⬇️
src/iter/traverser.rs	95.96% <0%> (+1.61%)	⬆️

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[thallada]

Rebased and added docs. Let me know if this needs anything else!

[jonhoo]

Excellent, thank you!