Improve generic parameter names

[Anders429]

Replaces cryptic letters (C, R, S, etc.) with full names for public-facing generic parameters. Closes #171.

The only one I didn't touch was System::run() and ParSystem::run(). I would like for those to look as unintimidating as possible to external users, and the effort to make it so is being tracked in #188.

[codecov]

Codecov Report

Merging #213 (0e4bf12) into dev (5c8411f) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##              dev     #213   +/-   ##
=======================================
  Coverage   96.94%   96.94%           
=======================================
  Files          85       85           
  Lines       14000    14017   +17     
=======================================
+ Hits        13572    13589   +17     
  Misses        428      428           

Impacted Files	Coverage Δ
src/entity/mod.rs	100.00% <ø> (ø)
src/system/schedule/mod.rs	100.00% <ø> (ø)
src/world/impl_clone.rs	100.00% <ø> (ø)
src/world/impl_debug.rs	0.00% <ø> (ø)
src/world/impl_default.rs	100.00% <ø> (ø)
src/world/impl_eq.rs	100.00% <ø> (ø)
src/entities/mod.rs	100.00% <100.00%> (ø)
src/world/entry.rs	96.08% <100.00%> (+0.06%)	⬆️
src/world/impl_serde.rs	99.25% <100.00%> (ø)
src/world/mod.rs	99.90% <100.00%> (+<0.01%)	⬆️

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