bank16k_low: Introduce 16K low common banking logic and use it

bank16k_low: Introduce 16K low common banking logic and use it

We need a different algorithm because we must keep the low page safe not the high
one. The standard bank16k grows by inserting pages before the existing repeating
top map so that common is preserved in high space. The 6502 has common in low
space so we must instead preserve the first page mapping.