fix: improve typing of flatten to infer return iterator type

[forivall]

Also add a test for flatten, based on flatten.test.ts

[codecov[bot]]

Codecov Report

Merging #22 into master will increase coverage by 1.22%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #22      +/-   ##
==========================================
+ Coverage   92.02%   93.25%   +1.22%     
==========================================
  Files           8        8              
  Lines         163      163              
  Branches       29       29              
==========================================
+ Hits          150      152       +2     
+ Misses         13       11       -2

Impacted Files	Coverage Δ
src/iterate.ts	89.36% <100%> (+2.12%)	:arrow_up:

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[cojack]

@felixfbecker is this PR waiting for something else?

[felixfbecker]

@cojack please read the discussion above.

[forivall]

@cojack thanks for the reminder! i forgot i had this one open. I'll get to it soon, and I can also set up dtslint or https://www.npmjs.com/package/tsd in a new PR.