Unify address decoding logic

[clydeshaffer]

Address decoding is duplicated between MemoryRead and MemoryWrite, which is becoming a bit of a pain as debug and diagnostic tools are added to the emulator and need to access that data differently than the game console itself would.

Requirements:

• Handle reads to stateful-read addresses without altering state
• Allow debug tools to read normally write-only registers
• Let debug tools access banked memory independently of banking register settings
• Consolidate address decoding to one place