This also adds tests for those functions. We have, furthermore, removed
some redundant (and non-externed!) functions from apple2.mem.c which was
the old bank switching code that got moved to apple2.bank.c.
There is one failing test, which I expect to fail at this point; that's
because we need to write a read/write map function for the stack and
zero page so that they use aux memory when BANK_ALTZP is on, and main
memory when not.
As the Apple II (or rather the 6502 chip) is little-endian, meaning the
least significant byte is the first byte you read going forward, rather
than the last byte.
This also consciously ignores tests for free() (handled implicitly in
all teardowns) and run_loop (for which I don't have a good notion to
test right now).
