Modular exponentiation primitive

[kwxm]

CIP-0109 proposes a new modular exponentiation builtin for Plutus Core. This will calculate a^k mod n for integers a and k and a positive integer n (not necessarily prime) and it should fail if k<0 and a is not invertible modulo n (ie, when gcd(a,n) > 1).

We need to do the following.

• [ ] Identify a library function that provides the required functionality.
• [ ] Add a new builtin called modExpInteger or similar to Plutus Core. This may require some wrapping of the library function to enforce the expected behaviour for non-invertible elements.
• [ ] Cost the new function. This may need some new costing types.
• [ ] Add a corresponding function to PlutusTx
• [ ] Add some tests, perhaps including one or more realistic use cases to check that the costs are acceptable.
• [ ] Add conformance tests
• [ ] Add e2e tests
• [ ] Add the new function to plutus-metatheory
• [ ] Add the new function to the Plutus Core specification.