Improve coverage for location: :keep
Swap variables to kernel context
Same module variables must be explicitly shared

Prior to this commit, the following code would work:

    defmodule Hello do
      defmacro write do
        quote do
          a = 1
        end
      end

      defmacro read do
        quote do
          a
        end
      end
    end

    require Hello
    Hello.write
    Hello.read

Although this is convenient, it has many issues:

1. It is not clear at any point the intent of sharing the variables
   in between the macros

2. Whenever macros from the same module are nested, there is a chance
   a variable used before and after the nesting to be overriden

We address this issue by only allowing variables explicitly marked
with var! to be shared. Here is how the example above should be updated:

    defmodule Hello do
      defmacro write do
        quote do
          var!(a, Hello) = 1
        end
      end

      defmacro read do
        quote do
          var!(a, Hello)
        end
      end
    end

    require Hello
    Hello.write
    Hello.read
Eval should take into account other contexts