Currently, the library attempts to execute some semihosting operations
in a completely interrupt-free context. It does this by using
`riscv::interrupt::free`, which saves and restores the `mie` field of
`mstatus`. As a result, attempts to initiate semihosting calls outside
of M mode cause illegal instruction exceptions.

This commit provides a solution, by requiring users to choose one of two
features, "machine-mode" or "user-mode", which will compile different
versions of the functions in src/export.rs that do and do not suspend
interrupts, respectively. Failure to do so will throw a compiler error,
unless the "no-semihosting" feature was enabled.

A "supervisor-mode" feature was left out as the `riscv` crate does not
yet have an equivalent of `interrupt::free` for supervisor mode.

CI now also checks builds with both of these features.

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.

Documentation and repository clean-up

This reverts commit f4c48301.

It has been decided that the CI will be left in for now, and updated to
work with this crate in the future.

The old Cargo.toml was a bit out-of-date. This commit adds the RISC-V
Team to the authors list, as well as bumping the version and edition to
1.59.0 and 2021.

The current repository CI was copied from cortex-m-semihosting, so it is
not relevant to this one. In the future, it may be worthwhile to add
some proper CI for this repo.

Original documentation was largely an exact copy of cortex-m-semihosting
at the fork. This commit fixes that, removing unnecessary references to
cortex-m-semihosting and clearing out the changelog so that it's
relevant to this repository instead.

Merge upstream cortex-m-semihosting

Without this the correct sequence is not triggered in OpenOCD and is
treated as a regular debugger break.

While here update to use asm!() syntax.

refs: http://www.keil.com/support/man/docs/armcc/armcc_pge1358787051159.htm

Stdio fixes after review

See #11.

drop write macros in favor of IO objects + fmt::Write

closes #4
closes #5
closes #6
closes #7
closes #11

watch tail -> tail -f

note that it's possible to skip the separate log file.

closes #9

Add `exit` and `report_exception` syscalls.

add a macro to write to the host stderr

also fix `write_all` logic when the a single write doesn't write all the bytes

fix typo in hprintln!