ponyc: Extremely poor performance and high memory usage when iterating a FileLines instance

Given the program:

use "files"

actor Main
  new create(env: Env) =>
    try
      var count: U32 = 0
      var path = FilePath(env.root as AmbientAuth, "./googlebooks-eng-all-1gram-20120701-0")?
      var file = OpenFile(path) as File
      for line in FileLines(file) do
        count = count + 1
      end
    end

Running on the data file https://storage.googleapis.com/books/ngrams/books/googlebooks-eng-all-1gram-20120701-0.gz (uncompress first)

You will see the program takes an extremely long time to iterate the file, moreover, the ram usage is more than I had expected: ~5GB from ~180 meg input file.

For comparison with python:

$ cat test.py
count = 0
for l in open("googlebooks-eng-all-1gram-20120701-0"):
	count += 1

$ time python test.py

real    0m2.092s
user    0m1.310s
sys     0m0.108s

$ time ./test-pony
<<Manually kill>>
real	0m18.638s
user	0m2.584s
sys	0m13.456s

Could reproduce with pony 0.18 on windows 10 and linux (xubuntu).