book/import#import-func-code lacks examples of importing FunC functions returning tuples

[anton-trunov]

As Tact does not have tuple types (at least yet), like FunC does, importing such functions is challenging for users:

For instance, how does one import a function like the following?

(int, int) derive_pub_key(priv_key: int);

An obvious attempt for the novice Tact programmer fails:

@name(derive_pub_key)
native derivePubKey(privKey: Int): (Int,Int);   // (Int, Int) is invalid Tact