Add cl-cancel (cancellation propagation with deadlines and timeouts)

[atgreen]

cl-cancel provides composable cancellation for Common Lisp, in the spirit of
Go's context package: cancellation tokens that propagate through a tree of
operations, with support for deadlines, timeouts, and cancel-aware streams.
Useful for coordinating shutdown and time-bounding of concurrent work.

Sources: https://github.com/atgreen/cl-cancel
Docs/README: https://github.com/atgreen/cl-cancel#readme
License: MIT

Suggested source.txt:
latest-github-release https://github.com/atgreen/cl-cancel.git

Dependencies (bordeaux-threads, atomics, precise-time) are already in Quicklisp.