Using some String functions adds 16.9MB to output #353

I am trying to compile the OT.Text module from https://github.com/jclem/ot_ex and am finding that even if I try to compile only the rather small OT.Text.Component module, the presence of String.slice/3 adds 16.9MB to the JavaScript output.

I have also found that the presence of String.split_at/2 adds the same amount. Simply commenting out the lines that include either String.slice/3 or String.split_at/2 reduces my output from 18MB to 1.1MB.

In order to check that I wasn't doing something wrong, I attempted to compile just this module:

defmodule Test do
  def split_abc do
    String.split_at("ABC", 2)
  end
end

Its output is also 18MB. What's causing this very large compilation size, and can it be fixed? This is with ElixirScript v0.30.0

btw I forgot to say, thanks for spending your time on ElixirScript, I'm super super super excited about it