Clean up code around Stream.zip/2
Clean up code around Stream.zip/2

This commit contains no functional changes, but it renames a bunch of
variables and functions to make their purpose clearer and adds a couple
of informative comments.

gfvcastro/elixir,kelvinst/elixir,gfvcastro/elixir,kelvinst/elixir,kimshrier/elixir,ggcampinho/elixir,lexmag/elixir,pedrosnk/elixir,beedub/elixir,antipax/elixir,pedrosnk/elixir,lexmag/elixir,ggcampinho/elixir,michalmuskala/elixir,kimshrier/elixir,elixir-lang/elixir,beedub/elixir,antipax/elixir,joshprice/elixir