Turned sim65 into a lightweight cc65 execution environment.

The sim65 source code has been a construction site for over a decade.
I was looking for a simple cc65 program execution environment for
regression tests. So I decided to re-purpose sim65 for that task by
removing about everything but the 6502 emulation.

There's no memory mapped i/o emulation whatsoever. Rather exit(),
open(), close(), read() and write() calls are supported by mapping
them through a thin paravirtualization layer to the corresponding
host os functions.

Note: The sim65 6502 emulation provides means to switch between
6502 and 65C02 emulation but currently there are no actual 65C02
opcodes implemented.