Add support for large messages

[evilrix]

Rather than having to verify a whole memory buffer in one go, allow chunks of
memory to be processed using begin, update and end verify functions. The begin
function starts a new verify process, returning an opaque context handle. The
update function can then be called multiple times with this context and blocks
of memory to process memory in chuncks. Finally the end function is called with
the context to end the verify process and get the result.

[immesys]

This is very useful, I added my own variant of this when I started using ed25519. I know reviewing crypto contributions is a pain but it would be nice to have this out the box.

[ghost]

thanks for the pr, I'll patch my source tree

[paulocoutinhox]

Hi,

It can be used inside a ESP8266 12F or Arduino or Both?

Thanks.

[rdebath]

If you're signing large amounts of data are you supposed to generate a SHA512 of the data you want to sign and then sign that 512 bit hash? Do the same (calculate the SHA512 of the input) then verify the hash at the other end.

If so the facilities for that already exist in the library, ie the SHA512 can be fed in blocks.

It should also be faster for signing as you only have to scan the file once, verifying the signature will, of course be exactly the same speed.