Fix a type specification.

When calling `stream_xor/3` it is possible to supply `iodata()` directly
and you don't have to convert to a binary first. Reflect this in the type
of the specification.
Introduce iodata() in the EQC tests.

Introduce a type for iodata() in the EQC tests. Use this type throughout
the tests in order to make sure we can supply iodata() in all places where
we claim we can supply iodata().