Improve audio

[Amjad50]

Partial fix for #9

General improvement for audio quality.

• Emulator uses a Digital-Analog-Converter (DAC) simulation to improve audio emulation quality.
• Emulator returns Stereo audio now (NES is mono, so its just copy of the same channel 2 times).
• Other code fixes.

There are still some emulation inaccuracies, for example in SMB2, when jumping the audio is being cut short for some reason.
This will require more work

[codecov]

Codecov Report

Attention: Patch coverage is 88.88889% with 7 lines in your changes missing coverage. Please review.

Project coverage is 82.17%. Comparing base (a89b8dc) to head (cc786f2).
Report is 5 commits behind head on master.

Files with missing lines	Patch %	Lines
plastic_core/src/apu2a03/channel.rs	91.17%	3 Missing ⚠️
plastic_core/src/apu2a03/mod.rs	88.88%	2 Missing ⚠️
plastic_core/src/misc.rs	0.00%	1 Missing ⚠️
plastic_core/src/nes.rs	90.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #46      +/-   ##
==========================================
+ Coverage   81.81%   82.17%   +0.35%     
==========================================
  Files          37       37              
  Lines        5352     5312      -40     
==========================================
- Hits         4379     4365      -14     
+ Misses        973      947      -26     

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