Add paragraph mode to String#each_line in mrblib

mruby/mruby#4511 demonstrated an infinite loop in `String#each_line` when
given an empty string separator. In MRI, an empty separator places
String#each_line in paragraph mode, where the String is separated on
successive runs of newlines. In paragraph mode, the String
`"abc\n\n\ndef\nxyz"` is split into `["abc\n\n\n", "def\nxyz"]`.

This commit makes the String#each_line implementation as close to
ruby/spec compliant as possible given the limitations of mruby core.
With this patch, the following specs fail for `String#each_line`:

- uses `$/` as the separator when none is given (can be fixed by
  aliasing and redefining the method to use $/ as the default value
  of separator in mruby-io)
- when no block is given returned Enumerator size should return nil
  (`Enumerator#size` is not supported on mruby)
- tries to convert the separator to a string using to_str (`String#to_str`
  is not implemented on mruby)

This patch has similar memory consumption compared to the prior
implementation and is takes 4x the time the prior implementation takes to
execute:

```console
/usr/bin/time -l ./bin/mruby -e '("aaa\n\nbbbbb\n\n\n\n\ncccc" * 100000).each_line("\n") { }';
```

0 files changed, 0 insertions, 0 deletions