Match APU buffering to CPU ticks

[Humpheh]

Previously the APU ran completely without knowledge of how fast the CPU was running. This PR makes it so that the APU is buffered with the number of CPU ticks. This fixes issues with complex sounds such as in the opening of Pokemon Yellow.

[codecov]

Codecov Report

Merging #29 into master will decrease coverage by 1.16%.
The diff coverage is 20.75%.

@@            Coverage Diff             @@
##           master      #29      +/-   ##
==========================================
- Coverage   71.61%   70.45%   -1.17%     
==========================================
  Files          21       23       +2     
  Lines        2463     2505      +42     
==========================================
+ Hits         1764     1765       +1     
- Misses        665      706      +41     
  Partials       34       34              

Impacted Files	Coverage Δ
pkg/gbio/iopixel/pixels.go	0.00% <0.00%> (ø)
pkg/apu/waves.go	5.88% <5.88%> (ø)
pkg/apu/channel.go	16.00% <16.00%> (ø)
pkg/apu/apu.go	31.28% <24.46%> (+0.30%)	⬆️
pkg/gb/gameboy.go	78.61% <33.33%> (-0.88%)	⬇️

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