Starting point for the new MMIO interface
Design doc:
https://docs.google.com/document/d/11qcGCWLne1wYvmtFaSrOKZt_vHxXrcWcZsdWJ-MJnyo/edit
The code is currently not used. Migration plan:
1. Implement MMIO access via MMIO::Mapping in parallel to the current
method.
2. Implement all existing MMIO handlers via the new interface.
3. Remove the old hwRead/hwReadWii/hwReadIOBridge code.
4. Implement JIT optimizations for MMIO accesses.