Refactor Cpu and System traits

[breqdev]

• Refactor out a Cpu trait from the Mos6502 struct to prepare for supporting other CPUs (Z80, etc)
• Remove the SystemInfo struct (previously only used to store the number of clock ticks) and standardize on using u64 for clock cycle counts
• Remove the WhateverBuilder unit structs, refactor the SystemBuilder trait into a BuildableSystem trait which is implemented for the system itself
• Add a get_cpu method to the System trait to allow for provided implementations to be added to System (e.g., upcoming work in trace generation in Add an option to produce a textfile trace of instructions executed #39)
• Add a cleanup method to the System trait (enables use of a buffered writer in Add an option to produce a textfile trace of instructions executed #39, improving performance)

[ava-silver]

lgtm! good refactor

[codecov]

Codecov Report

Attention: 81 lines in your changes are missing coverage. Please review.

Comparison is base (382f14b) 27.49% compared to head (688de5e) 27.40%.

Files	Patch %	Lines
src/cpu/mos6502/mod.rs	77.89%	21 Missing ⚠️
src/systems/pet/mod.rs	0.00%	7 Missing ⚠️
src/main.rs	0.00%	6 Missing ⚠️
src/systems/c64/mod.rs	0.00%	6 Missing ⚠️
src/systems/vic/mod.rs	0.00%	6 Missing ⚠️
src/memory/mos652x/pia.rs	0.00%	5 Missing ⚠️
src/platform/winit/mod.rs	0.00%	4 Missing ⚠️
src/systems/basic.rs	0.00%	4 Missing ⚠️
src/systems/easy.rs	0.00%	4 Missing ⚠️
src/systems/klaus.rs	40.00%	3 Missing ⚠️
... and 8 more

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #40      +/-   ##
==========================================
- Coverage   27.49%   27.40%   -0.10%     
==========================================
  Files          41       42       +1     
  Lines        8988     9014      +26     
==========================================
- Hits         2471     2470       -1     
- Misses       6517     6544      +27     

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