Add GitHub CI, remove broken Travis one

[Tachi107]

Since a commit recently broke tests, I thought it'd be useful to resurrect CI :)

I also had to stop passing -Wmaybe-unitialized unconditionally, as macOS' Xcode uses Clang.

[wwylele]

It looks like github doesn't run the test until this is merged, so I'll say this looks about right and merge it first

One thing that we lost from Travis is that there is a huge test set for almost every Teak instructions (that's what TEAKRA_TEST_ASSETS_DIR is for, which was stored in Travis cache). This test set needs to be re-generated from 3DS using the test generator in this repo. It would be great if we can add it back at some point

[wwylele]

Amending my previous comment: it looks like the test case file is still valid here https://github.com/wwylele/teakra/blob/705c0b342b79a94eeabf9b2f39e042f672bd628b/src/test_verifier/CMakeLists.txt#L21C8-L21C66 (and I just downloaded a copy myself) so we don't need to generate it from 3DS again