Apply fusion optimization to input iterator

[patrickroberts]

This change is made to expand the scope of a fused dispatch optimization based on the understanding that input iterators model the semantic requirement *i is equality-preserving, which is to say that two evaluations of this expression with the same input must have equal outputs.

This is why, for example, std::ranges::istream_view<Val> caches Val.

[coveralls]

coverage: 81.25% (+0.3%) from 80.934% — input-iterator-fusion-optimization into main

[foonathan]

I haven't looked into your implementation, but I can confirm that you are allowed to cache an input iterator: https://eel.is/c++draft/iterator.concept.readable#2

auto a = *it;
auto b = *it;
assert(a == b); // always true