NES: Rework CNROM, UNROM and UNROM-512 to properly handle NMIs.

[asiekierka]

Fixes https://github.com/llvm-mos/llvm-mos-sdk/issues/195 for these three mappers. I think this leaves only MMC3 as the remaining mapper without NMI handling (MMC1 and Action53 already had their own; NROM does not require it).

I've implemented my own approach to handling NMIs:

• _BANK_SHADOW is always written to before the mapper's MMIO register itself;
• _BANK_SHADOW is written to MMIO at an early stage of NMI, allowing developer-side consistency for NMI code written in C once the AXY calling convention is added.

For CNROM and UNROM-512, I've extended this with additional functionality:

• The graphical part of the register (CHR bank, mirroring) is now deliberately separated between "shadow" (written to the MMIO register on NMI or on user request) and the MMIO register.
• Writing to the "shadow" without writing to the MMIO register is called a "swap" - this will only apply the result on the next frame. This allows preparing a CHR bank change without affecting the currently drawn frame.
• Writing to the MMIO register without writing to the "shadow" is called a "split" - this will only apply the result until the next frame. This allows doing consistent mid-frame effects.

As this may not be immediately intuitive to someone tinkering with the code in the future, I have added a test for these two mappers which automatically validates it.

This required removing get_chr_bank on CNROM - well, it didn't strictly require it, but then it would require two RAM bytes instead of one; I expect the functionality to be sufficiently unnecessary as to not be worth the added cost. Likewise, the public API of UNROM-512 has changed; as neither API has yet shipped in a production build of the SDK, this is currently not a breaking change.

[mysterymath]

EDIT: I withdraw most of this; I noticed this was based off of jroweboy's work w/ Action53. I need to read that more carefully and fold this into my opinion.

[asiekierka]

Perhaps it'd help to get more eyeballs on the PR? There are a few NES LLVM-MOS-SDK users by now; they might have their own opinions to contribute.

[mysterymath]

Yeah, paging @jroweboy , if you get the chance.