Support returning literals in specs

[mkaput]

sometimes it could be handy to hide generation of constant literals inside unifex specs, for example to express void procedures, or known error atoms:

spec remove_stream(state, stream_id :: unsigned) :: nil
spec create() :: {:ok :: label, state} | {:error :: label, :failed}

on ambiguous cases, unifex should raise a compilation error, e.g.:

spec foo() :: {:error :: label, 8} | {:error :: label, 9}

[mkaput]

Currently as a workaround one may use :: label, though note it influences result creators' naming:

spec remove_stream(state, stream_id :: unsigned) :: (nil :: label)
spec create() :: {:ok :: label, state} | {:error :: label, :failed :: label}

[mickel8]

Moved to #61