* Sprinkle `const` around where appropriate
- This will make it easier to use `NDS` objects in `const` contexts (e.g. `const` parameters or methods)
* Remove the `const` qualifier on `DSi_DSP::DSPRead16`
- MMIO reads can be non-pure, so this may not be `const` in the future
* Reorganize namespaces
- Most types are now moved into the `melonDS` namespace
- Only good chance to do this for a while, since a big refactor is next
* Fix the build