Implement basic file io

[daniel-p-gonzalez]

e.g.

in_file = open('file.txt', 'r')
out_file = open('copy.txt', 'w')

for line in in_file:
  out_file.write(line)
end

# maybe copy-on-write to avoid this being necessary
# e.g. don't allow another actor to hold a reference, and close on leaving scope
in_file.close()
out_file.close()

[daniel-p-gonzalez]

Dupe of #20 (zenhub wasn't refreshing)