Unused functions with cyclic dependencies are now also warned as unused

Closes #1105
Mention IEx.Options in CHANGELOG
Rename Mix.SCM.equals? to Mix.SCM.equal?
Add String.starts_with? and String.ends_with?
Merge pull request #1113 from eproxus/string_end_begin_with

Add String.starts_with? and String.ends_with?
Remove *_table terminolgy in favor of setup/cleanup
Reduce atom footprint when compiling an elixir module