Previously we tolerated EOF as an alias for newline, but a file without
a newline at the end is a edge case primarily limited to contrived
examples in unit tests, and requiring it simplifies tasks such as code
generation in zclwrite, since we can always assume that every block item
comes with a built-in line terminator.
It no longer produces diagnostics, since that's redundant with the
diagnostics that Decode itself will produce, and it wasn't going to be
complete anyway due to our use of partial decoding and our inability
to thread through nested specs in child blocks.
