Replaced old CI with one using GitHub Actions (#4)

The old CI was meant for cortex-m-semihosting, so it would not have
worked with this repository, and it also used Travis CI, whereas
the new one uses GitHub Actions.

The CI runs `cargo check` for the following targets:
- `riscv32i-unknown-none-elf`
- `riscv32imc-unknown-none-elf`
- `riscv32imac-unknown-none-elf`
- `riscv64imac-unknown-none-elf`
- `riscv64gc-unknown-none-elf`

It also checks formatting and doc.