Allow usage of MillerLoopResult independently of the builtin apis

[dignifiedquire]

In the former implementation of pairing it was possible to use Fp12 directly to work with the intermediary results of miller loops, before doing the final accumulation. With the new API of MillerLoopResult and Gt this is impossible.

You can see an example of this here: https://github.com/filecoin-project/bls-signatures/blob/master/src/signature.rs#L158-L208, which I am trying to migrate to this crate.

[dignifiedquire]

Actually most of it is possible, I am just missing a way to create an empty MillerLoopResult I realized, so just implementing Default for it would probably be enough for my use case.

[str4d]

Yeah, impl Default for MillerLoopResult seems reasonable to me. Though now that Iterator::reduce is available (which doesn't require an initial accumulator value), is this necessary?