Moved SHA 512 work area to local stack

Moved SHA 512 work area to local stack

While some users could perhaps benefit from saving 640 bytes of stack
space by allocating the context statically, or in the heap, in practice
it's not he bottleneck.  Besides, putting the work area there actually
*increases* stack usage on signatures and signature verification, which
are the most stack hungry parts of Monocypher to begin with.

Better not try to be clever.