Add CI (& bors)

[jeehoonkang]

It'll be convenient to have a CI that automatically checks if each build is successful and passes the test. Fortunately, we may be able to use Travis CI or other free-of-charge CI services.

Also, consider using @bors-ng: https://bors.tech/ You don't need to worry about whether your merge introduces build or test failures.

cc @efenniht, who is actually porting the rest of C code to Rust.

[hkim15]

Welcome @efenniht to the project!