Each line

Improve IStream and Path for easiness of reading.

First, add new iterator `each_line` that lazily `read_line` of a stream.

Second, wrap some reading methods on `Path` so that the input of data from files is really easy.

~~~nit
var content = "/etc/issue".to_path.read_all

for l in "/etc/passwd".to_path.each_line do
   var fields = l.split(":")
   print "name={fields[0]} uid={fields[2]}"
end
~~~

TODO: better error management
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