rp/pio: write instr memory only from common

instruction memory is a shared resource. writing it only from PioCommon
clarifies this, and perhaps makes it more obvious that multiple state
machines can share the same instructions.

this also allows *freeing* of instruction memory to reprogram the
system, although this interface is not entirely safe yet. it's safe in
the sense rusts understands things, but state machines may misbehave if
their instruction memory is freed and rewritten while they are running.
fixing this is out of scope for now since it requires some larger
changes to how state machines are handled. the interface provided
currently is already unsafe in that it lets people execute instruction
memory that has never been written, so this isn't much of a drawback for now.

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