Remove semantically useless const qualifiers on returns and fix examples with the cleanup.

[FunMiles]

This should be an easy pull request. It's a simple series of clang-tidy flagged cleanups. They do create a strange issue with the examples, but it is because the examples had a confusing auto & where it should have been either const auto & or auto. All such instances have been addressed.

[lhog]

Looks like your version indeed produces less asm instructions: https://godbolt.org/z/6WGW7o