Add FuseSoC support

[olofk]

Adds support for using async_fifo in FuseSoC-based projects.

Also makes it possible to run async_fifo unit tests with different simulators. This has been tested with icarus, modelsim and xsim

To test this

1. Install and initialize fusesoc with pip install fusesoc and fusesoc init
2. Create an empty workspace directory and run all commands from here
3. Add svut as a FuseSoC library (requires Add FuseSoC support svut#18). fusesoc library add svut /path/to/svut if it's already on your disk, or fusesoc library add svut https://github.com/damofthemoon/svut to get the upstream version
4. Add async_fifo as a FuseSoC library fusesoc library add async_fifo /path/to/async_fifo
5. Run the simulation fusesoc run --target=sim async_fifo. This will use icarus by default. To change tool run with fusesoc run --target=sim --tool=$tool async_fifo, where $tool can be icarus, modelsim or xsim. vcs and rivierapro might work too. Haven't tested

To get VCD dumping or force timeout you can add --vcd or --timeout=some_value at the end of the command-line. To get all options for a specific target, run fusesoc run --target=sim async_fifo --help

[dpretet]

Thank you @olofk 👍 😊