Refactor IO and fix some small bugs

[rynorris]

• Refactor IO to allow us to pass in dummy IO modules in tests.
• Refactor Clock to not include any real-time timing. That's handled outside now and should make testing easier. It also means we now render the screen and process input at 30fps regardless of how fast the emulator is running. This makes running in slow-mo much more responsive.
• Fix some bugs with Sprite 0-hit.

[codecov-io]

Codecov Report

Merging #10 into master will increase coverage by 0.36%. The diff coverage is 28.57%.

@@            Coverage Diff            @@
##           master     #10      +/-   ##
=========================================
+ Coverage   93.73%   94.1%   +0.36%     
=========================================
  Files          30      30              
  Lines        4680    4732      +52     
=========================================
+ Hits         4387    4453      +66     
+ Misses        293     279      -14

Impacted Files	Coverage Δ
src/emulator/memory.rs	97.22% <ø> (+0.34%)	:arrow_up:
src/emulator/ppu/test/background.rs	100% <ø> (+45.71%)	:arrow_up:
src/emulator/io/sdl.rs	0% <0%> (ø)	:arrow_up:
src/emulator/clock.rs	97.26% <100%> (+0.1%)	:arrow_up:
src/emulator/ppu/mod.rs	88.32% <81.25%> (+1.34%)	:arrow_up:

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