Define Instance, Module, etc. inside Wasmer module

[irxground]

PHP has a namespace feature but Ruby does not. In Ruby, we define classes inside of module instead.

module Wasmer
  class Instance
    def initialize
      # ...
    end
  end
end

instance = Wasmer::Instance.new()

I changed to define classes inside the Wasmer module to prevent the global namespace from being polluted with generic name such as Module.

[Hywan]

Thanks for the PR! It's much better like this :-).