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Now simulated 'Memory' isn't just a single flat array. Instead it knows
about segments and VMAs.
The code segment will always be first, and the data/heap segment will always
be second. The brk() syscall knows about the data segment.
One nice side-effect is that I no longer need to mess with Memory initialization
regardless of where I place my segments.
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Include LEA (load effective address) in the SubX subset of x86 ISA.
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Streamline the factorial function; we don't need to save a stack variable
into a register before operating on it. All instructions can take a stack
variable directly.
In the process we found two bugs:
a) Opcode f7 was not implemented correctly. It was internally consistent
but I'd never validated it against a natively running program. Turns out
it encodes multiple instructions, not just 'not'.
b) The way we look up imm32 operands was sometimes reading them before
disp8/disp32 operands.
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The new example ex9 doesn't yet work natively.
In the process I've emulated the kernel's role in providing args, implemented
a couple of instructions acting on 8-bit operands (useful for ASCII string
operations), and begun the start of the standard library (ascii_length
is the same as strlen).
At the level of SubX we're just only going to support ASCII.
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