sign-ed25519: Drop some uses of libsodium

sign-ed25519: Drop some uses of libsodium

This adds some defines for ed25519 key sizes and drops uses
of the libsodium defines for these, as well as replacing sodium_bin2hex
use with ot_bin2hex. Some code that wes optionally built before are now
always built.

The goal for this is to support both libsodium and openssl.

Also fixes return value of _load_pk_from_stream(). It used
to always return FALSE.