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)
Each supported assembler has an IGenerator interface and an
IAssembler interface. They're still two separate classes, but now
both are implemented in the same source file. (They'll probably
stay separate classes, since the two have little interaction.)
I'm keeping the "Asm*" filename. Seems the more natural fit.
Also, changed AssemblerInfo to try to get all assembler-specific
stuff into a single table.
