This is more flexible than the old acorn26 bus_space, which means that single
read/write operations are slower, but multi and region operations have the
potential to be faster, and particularly insane podules might be supportable.
In theory the acorn32 mainbus and podulebus code ought to be shareable, but
acorn26 needs the extra overhead of saving R14_svc on some operations so
I've made my own version for now. Also the acorn32 bus_spaces are a mess.
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