Overhaul schedule module.

[Anders429]

Completely removes the run-time builder system for schedules. Instead, type-level recursion is used for defining stages. Stages are defined respective of a registry (this was the only way I could get around the need for negative trait bounds, since creating an inverse set of a set of views using a registry can simulate negative trait bounds), which means that stages are actually assembled from the list of tasks when World::run_schedule() is called. This should, however, be really cheap, since the type system has already defined the stages. The only work needed to be done at runtime is to reference the tasks into their stages.

Therefore, schedules are now defined as heterogeneous lists of tasks. This eliminates the need for the complicated stages! macro, allowing us to define a simple heterogeneous list like is done elsewhere.

Future work may involve a Schedule! macro for generating the type of a schedule. This can make it easier for users to store schedules.

Honestly, I'm pretty proud of the type-level recursion stuff going on here. It came out super clean. It basically does the same thing that was being done at run-time previously, but all using the trait system.

Fixes #138.

[Anders429]

It appears a change in the latest Rust nightly has caused the trybuild tests to fail. I will add a fix for those to this PR.

Edit: Yep, the culprit is this PR.

[Anders429]

Fixed the test output. Not sure why result/comma_alone.stderr seems to produce a different error than the others, but I think it's acceptable.

[codecov-commenter]

Codecov Report

Merging #142 (5e79390) into dev (1396d59) will decrease coverage by 0.26%.
The diff coverage is 74.10%.

@@            Coverage Diff             @@
##              dev     #142      +/-   ##
==========================================
- Coverage   95.08%   94.81%   -0.27%     
==========================================
  Files          73       73              
  Lines        9991    10223     +232     
==========================================
+ Hits         9500     9693     +193     
- Misses        491      530      +39     

Impacted Files	Coverage Δ
src/system/mod.rs	0.00% <ø> (ø)
src/system/schedule/mod.rs	71.34% <71.22%> (-28.66%)	⬇️
src/system/schedule/stager.rs	81.25% <81.25%> (ø)
src/system/schedule/scheduler.rs	100.00% <100.00%> (ø)
src/system/schedule/sealed.rs	100.00% <100.00%> (ø)
src/system/schedule/stage.rs	100.00% <100.00%> (ø)
src/system/schedule/stages.rs	100.00% <100.00%> (ø)
src/system/schedule/task/sealed.rs	100.00% <100.00%> (ø)
src/world/mod.rs	100.00% <100.00%> (ø)

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.