Circular allocator

[tilk]

Sometimes a FIFO-like allocation system is needed without an actual FIFO. This PR factors out the allocator of BasicFifo and extends it with multi-allocation.

TODO:

• Return all allocated/deallocated IDs for convenience
• Docstrings
• Tests

[awariac]

Return all allocated/deallocated IDs for convenience

Would be helpful to add extra option to enable logging of each allocation/deallocation, as similar mechanism is used e.g. in Coreblocks CRAT.

[tilk]

I think that it's probably better not to include logging here. Logging messages are nicer when more domain-specific information is available, and it always can be added at the use site.

I'm experimenting with the docstring format for methods. I think that it's better to use shapes rather than types after the colons, as this is the information you actually need when interfacing with a method.