Before we masked all except the removed cell rows, then unmasked all
rows on every removal attempt.
Now, we mask all except the removed cell rows once each attempt in an
order such that we can simply unmask the last (values - 1) rows to try
the next cell. An unmask/remask cycle is only required when a removal
attempt fails.
threads.js has better support for modules - no need to give a
project-relative path to the worker file, which complicated the build.
Add rudimentary thread pooling w/ execution timeout.
Moves sudoku generation off of the main thread. Allows for multiple
generation requests (up to the number of physical CPU cores) to be
served in parallel. Uses the "physical-cpu-count" node package to
determine the numer of physical CPUs of the host.
Also introduces a timeout which causes too-difficult (or impossible)
generation requests to fail if they take more than 5 seconds to
complete. In effect soft-capping the complexity of generated puzzles.
