Add `BufferedReader::linesIterator()`

[fredemmott]

Also:

• specialize readLine more
• make nullable vs exception-throwing versions

Iterating while ($reader->isEndOfFile()) has several edge cases:

• file already closed (especially STDIN)
• foo\nbar should be vec["foo", "bar"], not "foo" followed by EPIPE

readUntil still does exactly what it says on the tin.

The new behavior matches both C Rust IO. It's different from python, which addresses this by including the "\n".

Going for this approach instead of the python approach as I expect most users to just Str\strip_suffix()-away the trailing \n and not handle the empty string.

Providing a higher-level API seems to be safer.

[facebook-github-bot]

@fredemmott merged this pull request in hhvm/hsl-experimental@03ed30db1de1dbac8996b74cb8bfa4ec65ad528e.