Macro for defining the type of a schedule.

[Anders429]

Fixes #145.

This adds a Schedule! macro, used to define the type of a schedule. The capitalization of this macro follows the pattern being established by #141.

Also fixes an internal issue with some macros doc tests being run multiple times. Not a big issue, but this ensures that when one of these macro tests fails, it will only be indicated in the test results a single time.

[codecov-commenter]

Codecov Report

Merging #146 (e6953f9) into dev (fda6fe7) will decrease coverage by 0.00%.
The diff coverage is 33.33%.

@@            Coverage Diff             @@
##              dev     #146      +/-   ##
==========================================
- Coverage   94.81%   94.81%   -0.01%     
==========================================
  Files          73       73              
  Lines       10223    10222       -1     
==========================================
- Hits         9693     9692       -1     
  Misses        530      530              

Impacted Files	Coverage Δ
src/registry/debug/sealed.rs	0.00% <0.00%> (ø)
src/system/mod.rs	0.00% <ø> (ø)
src/system/schedule/mod.rs	71.34% <ø> (ø)
src/world/mod.rs	100.00% <ø> (ø)
src/registry/sealed/storage.rs	99.92% <100.00%> (-0.01%)	⬇️

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[Anders429]

Miri failures are definitely a case of #82. As mentioned in that thread, rayon#952 indicates that this is actually caused by rayon itself, and has actually not caused any issues in practice.

Not sure if the best move is to just disable rayon in the miri tests or what. The problem only seems to occur in the schedule tests. For now, I'll just leave it and merge this PR.