Allowing binding to list its local variables

Before this patch:

    def get_all_local_variables(bind)
      lvars = bind.send(:local_variables)
      # `lvars` should equal [:x, :y], but equals [:bind, :lvars]
      lvars.map {|name| bind.local_variable_get name }
    end
    x = 1
    y = 2
    get_all_local_variables(binding) # NameError: local variable `bind' not defined for #<Binding:0x0>

After this patch:

    def get_all_local_variables(bind)
      lvars = bind.local_variables
      # `lvars` equals [:x, :y]
      lvars.map {|name| bind.local_variable_get name }
    end
    x = 1
    y = 2
    set_everything_but_x_to_5(binding) # => [1, 2]

bind.send(:local_variables) was a global method that used the current
stack frame regardless of binding. Now that we have
`Binding#local_variable_get` and `Binding#local_variable_set` the API
feels very strange without `Binding#local_variables` working similarly.