[WIP] Implement CNF support for non-CAFFY functions

Just a proof of concept. Commit message below.

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This patch allows adding non-CAFFY functions to compact regions. Because non-CAFFY functions don't have any garbage collected references other than stuff in the payload this operation should be safe.

Other changes:

• Renamed HALF_W_ Cmm macro to TO_HALF_W_. This is more consistent with other TO_ macros.
• HALF_W_ is now a Cmm type macro for bits16 on 32-bit, bits32 on 64-bit.

mentioned in issue #17734

I wrote about why this is not a good idea here.

Closing.

closed