Let's require symbol keys in hashes

[wasnotrice]

Currently, Brown Shoes stringifies keys of hashes before processing them. Red Shoes just expects symbol keys and ignores what it doesn't expect. I think purple and green also deal only in symbols. Stringifying all of the keys adds complexity without really adding any benefit.

This example, from the manual, works in Red Shoes:

Shoes.app do
   @s = stack { background green }
   @s.style(:width => 400, :height => 200)
 end

but with stringified keys, it doesn't:

Shoes.app do
   @s = stack { background green }
   @s.style('width' => 400, 'height' => 200)
 end