a 32-bit arch on a 64-bit platform, eg NEON on AARCH64.  It's probably
more useful for cross-platform testing, though.

The breakdown is as follows:
* decaf_bool_t, decaf_word_t and decaf_error_t are as defined in the API.
* DECAF_WORD_BITS is the size of a decaf_word_t.
* decaf_word_t is used for scalars, so on every curve the scalar impls are the same
(i.e. they follow the API's word size).
* SC_LIMB macro always takes a 64-bit word.

* non-prefixed word_t, mask_t, etc are as defined by the per-curve arch.
* ARCH_WORD_BITS is the size of a word_t.
* word_t is used for gf elements, so the curves may have different guts.