Bus.Write() doesn't ignore backendFor errors.

This is relevant when attempting to write to memory which isn't mapped
to a backend by the address bus.
ADC, ASL, BCC, DEC instructions.

Also additional LSR and ROL addressing modes.
SBC instruction; subtract with carry.
Ssd1306 pixel counter capped to 128*64 (matches GDDRAM).

Display is actually 128x32 but the the driver supports up to 128*64 via
1 KiB of RAM.
Ssd1306 serves screen as PNG over HTTP.
Ssd1306 HTTP client specifies refresh; e.g. URL?refresh=0.2
Delete unused srName() from cpu.go

If it comes back in future, it should probably be a const [8]string.
ASL tidied, supports all addressing modes.
LSR tidied, supports all addressing modes, correct carry.
ROL irrelevant TODO removed.
Instruction and Iop have nicer String().
Cpu.Reset leaves most registers untouched as per 65C02 manual.
Tidied Cpu.statusString() a little.
CPU instruction method code comments from WDC manual.
Iop moved from cpu.go to instruction.go, commented.
Instruction mnemonic constants not exported.
instructionNames array instead of giant switch.
Instruction renamed to OpType.
Iop renamed to Instruction.
findInstruction() replaced by optypes map.