Port ci from travis to github actions

[jhorstmann]

This sets up github actions workflows that should run the same tests as were previously setup for travis ci. Luckily none of the shell scripts needed changing, only a new set of yaml files.

There are a few FIXME issues, where I could use some help or which could maybe be solved in followup:

• The toolchain version is currently pinned, this should probably be reverted in Stop using old-style simd_shuffle #350
• Examples are disabled because they were running too long. Maybe this is only a single example which can be excluded or whose parameters can be changed. I was not patient enough for the scalar fannkuch example to finish and see how long it actually takes.
• Docs are not yet published, there are some travis specific environment variables in dox.sh that I haven't looked into yet
• The ispc feature for the benchmarks is currently disabled, because I haven't found an easy way to add the binary to the PATH
• The required jobs should be setup using the github ui, I think this requires the jobs to be run once in the master branch

There is a mostly unrelated change in the instruction verification tests for vector rotates, that turned out to be a red herring. The verify tests for i586 and i686 were failing because the verifier did not expect a call instruction in the code. That call instruction however was only needed to support relocations, the code was getting inlined correctly. However, I think the target features are more correct after the change, the 128bit type m64x2 should require avx512vl, the other two tests use 512bit vectors where avx512f should suffice.

[jhorstmann]

The last run of the actions can also be seen in my fork: https://github.com/jhorstmann/packed_simd/actions
Would be nice if these could be somehow compared with the last successful run in travis. Especially the wrong results in some of the mips or powerpc tests are a bit worrying.

[workingjubilee]

Oh, this looks great! Thank you!

[workingjubilee]

Oof, this has some errors on master, judging by https://github.com/rust-lang/packed_simd/actions/runs/5673019417

[workingjubilee]

#355 gets us half of the way there.