Limit instructions per second

[craigthomas]

This PR introduces a new way to add CPU operation delay into the emulator. Instead of using a sleep delay, we now count operations per second, and limit the number that are allowed to execute. This speeds up the main loop since the overhead of starting a delay operation was quite high. The new instruction is introduced with the --ticks switch on the command line.

[codecov]

Codecov Report

Attention: Patch coverage is 70.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 94.71%. Comparing base (b802ca0) to head (cadd696).
Report is 6 commits behind head on main.

Files with missing lines	Patch %	Lines
chip8/cpu.py	77.77%	2 Missing ⚠️
chip8/emulator.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #50      +/-   ##
==========================================
+ Coverage   92.87%   94.71%   +1.83%     
==========================================
  Files           4        4              
  Lines         660      700      +40     
==========================================
+ Hits          613      663      +50     
+ Misses         47       37      -10     

Files with missing lines	Coverage Δ
chip8/emulator.py	0.00% <0.00%> (ø)
chip8/cpu.py	98.58% <77.77%> (+2.13%)	⬆️

---

Continue to review full report in Codecov by Sentry.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 460ac4f...cadd696. Read the comment docs.