partially address D-Scanner warnings in the whole project

[ghost]

Mostly trivial excepted that the signature of a function changes (Scanner.scanPlainSpacesToSlice).

[codecov[bot]]

Codecov Report

Merging #170 into master will decrease coverage by 0.02%. The diff coverage is 94.44%.

@@            Coverage Diff             @@
##           master     #170      +/-   ##
==========================================
- Coverage   93.38%   93.36%   -0.03%     
==========================================
  Files          31       31              
  Lines        4340     4338       -2     
==========================================
- Hits         4053     4050       -3     
- Misses        287      288       +1

Impacted Files	Coverage Δ
source/dyaml/dumper.d	89.47% <ø> (ø)	:arrow_up:
source/dyaml/test/emitter.d	93.93% <100%> (ø)	:arrow_up:
source/dyaml/loader.d	90.9% <100%> (ø)	:arrow_up:
source/dyaml/constructor.d	89.26% <100%> (ø)	:arrow_up:
source/dyaml/parser.d	97.73% <100%> (ø)	:arrow_up:
source/dyaml/test/common.d	83.67% <100%> (-0.33%)	:arrow_down:
source/dyaml/test/constructor.d	97.27% <100%> (ø)	:arrow_up:
source/dyaml/serializer.d	95.71% <100%> (ø)	:arrow_up:
source/dyaml/scanner.d	97.84% <100%> (ø)	:arrow_up:
source/dyaml/reader.d	97.97% <100%> (ø)	:arrow_up:
... and 2 more

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[Herringway]

Looks fine to me. The mark argument is only used for errors and there are no possible error states for this one.