Tune up some elementary funcs

[ckormanyos]

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[ckormanyos]

Still working on igamma_inv, which is the last specfun exhibiting edge-case problems.

[ckormanyos]

Updated branch protection in CI

[ckormanyos]

Ok the cpp_double_fp_backend template has passed all tests, including specfun. This is a good result, indicating that this type is relatively well-behaved.

Many thanks to all who helped get to this point @sinandredemption for excellent work in the GSoC, @cosurgi for mentoring this project and @jzmaddock for patiently answering our questions and helping us get to this point.

This point is a stable starting point for turning the cpp_double_fp_backend template into a Multiprecision type. We have a few TODOs, which I will formulate as a (TBD) list somewhere. I'm still looking for optimal add/sub/mul/div and we have a lot of edge-case checking and performance and constexpr-versus-runtime stuff to do.

@cosurgi this thing is stable enough for you to play around with.

Cc: @mborland