Merge neighboring identical tags in Owl.Data.from_ansidata/1 & increase coverage

[zachallaun]

This PR does two things:

1. When converting ansidata to Owl data, merge neighboring identical tags when recursively constructing the data. This is especially helpful when dealing with charlists:

# before
iex> Owl.Data.from_ansidata(IO.ANSI.format([:red, 'Hello']))
[[[[Owl.Data.tag(72, :red), Owl.Data.tag(101, :red)], Owl.Data.tag(108, :red)], Owl.Data.tag(108, :red)], Owl.Data.tag(111, :red)]

# after
iex> Owl.Data.from_ansidata(IO.ANSI.format([:red, 'Hello']))
Owl.Data.tag([[['He', 108], 108], 111], :red)

2. Adds missing test coverage referenced here.

[fuelen]

Thanks!