Public utility methods for stringify/symbolize keys

Sometimes I want to stringify or symbolize a key in a Hash without the need for a subclass, like [Kitchen::Util](https://github.com/test-kitchen/test-kitchen/blob/17ca05a88441e3897f828f39702452d19ba893a5/lib/kitchen/util.rb#L57-L89):
```ruby
symbolized_hash = Kitchen::Util.symbolized_hash(hash)
stringified_hash = Kitchen::Util.stringified_hash(hash)
```

This was basically implemented already by the `Hashie::Extensions::StringifyKeys` and `Hashie::Extensions::SymbolizeKeys`, but using protected methods that only worked as a mixin and not as a utility method that accepts a hash. This PR changes that so you can stringify/symbolize without including those modules:

```ruby
symbolized_hash = Hashie::Extensions::SymbolizeKeys.symbolize_keys(hash)
stringified_hash = Hashie::Extensions::StringifyKeys.stringify_keys(hash)
```