It seems just easier than having the copies in the current way.
I don't think the `static inline` were helping us anyway,
except for avoiding KR_EXPORT in some cases.

Still, differences when copying:
 - we use plain memset() in the implementation
   (no motivation here to use the complex memzero() approach)
 - we expose mm_malloc(), as we've been referring to it
 - we KR_EXPORT some of the functions (for lua and modules)