Add specs for ARGF.{each_line,lines,each}
Fix ARGF.{each_line,lines,each} with separator
Simplify ARGF.{chars,bytes}
Write specs for ARGF.{each_codepoint,codepoints}
Implement ARGF.{each_codepoint,codepoints}
Merge pull request #2493 from kachick/argf-enums

Fix ARGF.{each_line,lines,each}. And implemented ARGF.{each_codepoint,codepoints} for 2.0