README updates.
go6502 exports Address (was address).
via6522 package extracted from go6502.
OpType passed by value not pointer.
Instruction struct embeds OpType struct.
Cpu struct exports & documents registers.
OpType.Name() exported, was name().
debugger package extracted from go6502.

go6502 now expects Monitor interface, rather than a Debugger concrete
type, so that go6502 doesn't circular-import debugger.