Use checked pointers in all function replacements.

Some games are still hitting bugs and there's no way to disable this "fast
memory" without losing graphical effects.  Better to do it evenly than
weird hacks and extra logging/checks.
Disable a bunch of function replacements.

These are just for speed, let's turn them off.  Using a flag because:
 * I think there's still some issue with savestates, not sure.
 * We might swap this flag to a separate option.
Use a lookup for function replacements.

Ought to be a tiny bit quicker.
Don't zero out downcount in replacements.

It doesn't write out js.downcountAmount in any of these cases, so zeroing
it is wrong.
Merge pull request #6680 from unknownbrackets/replace-funcs

Disable most replacements and use checked mem access in them