Introduce mpz_import

[nordlow]

Further, after having read

https://code.dlang.org/packages/cerealed

and

https://forum.dlang.org/post/mailman.1263.1527468959.29801.digitalmars-d-learn@puremagic.com

I conclude that we should rename to to serialize and add a note in the docs of serialize that it calls mpz_export under the hood. Ok?

[basiliscos]

Yes, please, go ahead with serialize.

ubyte[] data = prime.to!(ubyte[])(...);

I wasn't able convert the current signature to that's one. I will be glad to see how to do that properly :)

[nordlow]

Merged, commented and tagged! Thanks!