Drop 384-bit multiplication impl with `slower_smaller_binary`

Drop 384-bit multiplication impl with `slower_smaller_binary`

...instead using the 512-bit multiplication impl and dropping the
resulting high bytes. This is much slower, but very few zones
actually use secp384r1 so in practice this shouldn't really matter.

This is a less than 5% reduction in performance and reduces code
size for the crypto module from 27.4KiB to 26.7KiB.