Prefix all warnings with [warning]

We expose :elixir_errors.warn/2 and :elixir_errors.warn/3
The first is to be used when we have detailed caller information,
such as from a stacktrace. The second is useful when our only
context is a line number and filename.

Note also that the test for module redfinition is performed in
helpers_test.exs rather than in warning_test.exs Perhaps this is
something that should be moved.