Reduced use of eval in RubyGems

The uses of eval found were needless as other, more expressive methods
of meta-programming are also available.

Removed the private class method Gem::each_load_path as it is no longer
used by RubyGems and was not callable.
Clear spec and files when verification fails

While a user must use the accessors, if the gem is unsigned you
shouldn't be able to extract information from it for extra protection.