Move the API definitions to a new package, CheckedArithmeticCore

[kimikage]

Closes #6

This also add support for div and % in @checked. (Fixes #4)
This switches the CI from Travis to GitHub Actions, and adds Julia v1 and 32-bit x86 to the test target.

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[timholy]

Thanks!

[kimikage]

There is still work to be done within CheckedArithmeticCore, but I think we need to register only CheckedArithmeticCore first, as it is a new package and registration will take 3 days.

I don't have any experience with registering a package in a subdirectory, is the following method available?
https://github.com/JuliaRegistries/Registrator.jl#registering-a-package-in-a-subdirectory

[timholy]

I've only done it once, for SnoopCompile, and that was before that was available. So no clue, but seems worth trying.

[kimikage]

It seems to be working well. It's great.
JuliaRegistries/General#21528