Crypt::DH replacement that uses GMP directly

 Crypt::DH::GMP is a (somewhat) portable replacement to Crypt::DH, implemented
 mostly in C.
 .
 In the beginning, there was Crypt::DH. However, Crypt::DH suffers from a
 couple of problems: GMP/Pari libraries are almost always required; Crypt::DH
 additionally has Math::BigInt in between GMP. Instantiating an object
 comes with a relatively high cost, and if you make many computations in one
 go, your program will suffer dramatically because of this.
 .
 Crypt::DH::GMP attempts to alleviate these problems by providing a
 Crypt::DH-compatible layer, which, instead of doing calculations via
 Math::BigInt, directly works with libgmp in C.
 .
 This means that essentially 2 call stacks worth of expensive Perl method
 calls are eliminated and also only load 1 (Crypt::DH::GMP) module instead of
 3 (Crypt::DH + Math::BigInt + Math::BigInt::GMP) is needed.
 .
 These add up to a fairly significant increase in performance.