Misc. stdlib improvements

[perimosocordiae]

The main contribution here is the file.Lines scope, which makes it easy to iterate over the lines of a file. I also did some cleanup of the core.for scope, which no longer requires exporting its state struct.

Known issues:

• we don't appear to ever destroy the scope state object, so we never free the line buffer.
• it would be nice to have a way to compose the file.With and file.Lines scopes, something like:

file.WithLines("foo.txt") each [line: []char] {
  // operate on line
} error [e: errno.error] {
  // handle file open/read errors
}