Ruby 3 compatibility

[katafrakt]

Due to some changes in Ruby 3, rb-gsl does not compile for this version. This PR:

• Adds 3.0 and all other minor versions to Travis
• Changes EXTERN to extern
• Solves issues around RBGSL_SET_CLASS0 and subtle change of behaviour in Ruby 3
• Takes care of Ruby 3 redefining memcpy, which broke some macros

[katafrakt]

Interesting thing: the tests on CI and my local Linux machine passed, but when I ran it on Mac, it failed and I had to add extra commit ffad864. This probably mean that CI could be better configured (perhaps it'd be enough to also run tests on MacOS) - but first the project needs to transition from travis-ci.org to travis-ci.com.

[david-macmahon]

FWIW, I think there was some kerfuffle recently about Travis changing their policies regarding "free" use for open source projects. A number of projects switched to GitHub Actions for CI in response to that. I'm not sure how relevant that is here, but thought I'd mention it.

[prasunanand]

Hi @katafrakt Would you like to be a maintainer of rb-gsl ?

cc: @v0dro @pjotrp @mrkn @translunar

[katafrakt]

@prasunanand Not gonna lie: I know close to nothing about GSL, only I use it as dependency of classifier-reborn. But I could help on the Ruby side I guess, if you need help.