replace Mark-esque bits of Scanner.SimpleKey with an actual Mark instead

[Herringway]

No description provided.

[codecov]

Codecov Report

Merging #321 (3d2e67d) into master (a0d7705) will not change coverage.
The diff coverage is 100.00%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #321   +/-   ##
=======================================
  Coverage   95.35%   95.35%           
=======================================
  Files          29       29           
  Lines        4496     4496           
=======================================
  Hits         4287     4287           
  Misses        209      209           

Files Changed	Coverage Δ
source/dyaml/scanner.d	99.65% <100.00%> (ø)

... and 2 files with indirect coverage changes

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