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.
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