ddar: fix syntax that errors on Ruby 2.1
Push expand_path call down into FromPathLoader
Eliminate some Pathname -> String -> Pathname conversions
Pass path into the Formula constructor

When the path argument to the Formula constructor is omitted, the
instance's path attribute is created using the Formula.path class
method. However, we have already done this work, so we can just pass it
into the constructor.

This translates to one less call to Pathname#to_s per formula, or about
2600 calls when running `brew readall`.