Clean up Dagon API a bit.

[non]

• Introduce Cache analog to HCache
• Rename ExpressionDag to Dag
• Move DependantGraph to tests, rename to SimpleDag
• A bit of other code clean up
• Some extra docs

[codecov-io]

Codecov Report

Merging #7 into master will decrease coverage by 1.97%.
The diff coverage is 74.28%.

@@            Coverage Diff            @@
##           master      #7      +/-   ##
=========================================
- Coverage   86.38%   84.4%   -1.98%     
=========================================
  Files          12      12              
  Lines         191     186       -5     
  Branches       14      13       -1     
=========================================
- Hits          165     157       -8     
- Misses         26      29       +3

Impacted Files	Coverage Δ
.../src/main/scala/com/stripe/dagon/PartialRule.scala	100% <ø> (ø)	⬆️
core/src/main/scala/com/stripe/dagon/Id.scala	100% <ø> (ø)	⬆️
core/src/main/scala/com/stripe/dagon/Dag.scala	93.93% <100%> (ø)
core/src/main/scala/com/stripe/dagon/Memoize.scala	100% <100%> (ø)	⬆️
core/src/main/scala/com/stripe/dagon/HCache.scala	57.14% <25%> (-9.53%)	⬇️
core/src/main/scala/com/stripe/dagon/Cache.scala	57.14% <57.14%> (ø)
core/src/main/scala/com/stripe/dagon/Rule.scala	75% <71.42%> (ø)	⬆️
core/src/main/scala/com/stripe/dagon/HMap.scala	75% <75%> (-2.78%)	⬇️

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