Let functions return "impl Iterator" to hide iterator structs

[mgeisler]

This slims down our public API a little by hiding the concrete type of iterator returned. This will hopefully make the API easier to understand.

[codecov-commenter]

Codecov Report

Merging #201 into master will not change coverage.
The diff coverage is 33.33%.

@@           Coverage Diff           @@
##           master     #201   +/-   ##
=======================================
  Coverage   92.44%   92.44%           
=======================================
  Files           4        4           
  Lines         397      397           
=======================================
  Hits          367      367           
  Misses         30       30           

Impacted Files	Coverage Δ
src/lib.rs	89.53% <33.33%> (ø)

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