DCI is handled with the ".shift" pseudo-op. The .null, .ptext,
and .shift operators all work correctly with escaped characters,
so we no longer redo those.
This was the result of the earlier change to eliminate "reverse DCI"
strings. On further examination, it doesn't seem like we can do
much better than a hex dump without more work than the situation
merits. So hex dump it is.
Most tests pass, but 2007-labels-and-symbols fails because the
expressions recognized by 64tass don't match up with either of the
other assemblers.
This is currently using a workaround for the local label syntax.
64tass uses '_' as the prefix, which is unfortunate since SourceGen
explicitly allowed underscores in labels. (So does 64tass for that
matter, but it treats labels specially when the '_' comes first.)
We will need to rename any non-local user labels that start with '_'.
(issue #16)
