Move SassScript parse nodes into Sass::Script::Tree.
Rename Sass::Script::Literal to Value.
Move SassScript value classes to lib/sass/script/value.

Also rename Sass::Script::Value to Sass::Script::Value::Base.
Attach source range information to SassScript Lexer tokens.
Improve List#inspect
Clean up source range assignment in the SassScript parser.
Make sure Lexer returns the correct line/offset.
Fix a typo in an error message.
Add Literal classes to wrap Values in parse trees.
Remove Sass::Script::Value::Base from the Sass::Script::Tree::Node hierarchy.
Add redirects for the old Script names.
Don't print a warning for old Sass::Script constants for now.

After the next release, we'll add a warning; after the release after that, we'll
remove them.
Add a changelog entry about the script refactor.