make Scanner a struct

[Herringway]

I was overthinking this one. Just needed to add a postblit.
For #215.

[codecov]

Codecov Report

Merging #219 into master will decrease coverage by 0.01%.
The diff coverage is 91.66%.

@@            Coverage Diff             @@
##           master     #219      +/-   ##
==========================================
- Coverage   95.23%   95.22%   -0.02%     
==========================================
  Files          28       28              
  Lines        4137     4148      +11     
==========================================
+ Hits         3940     3950      +10     
- Misses        197      198       +1

Impacted Files	Coverage Δ
source/dyaml/scanner.d	98.33% <ø> (ø)	⬆️
source/dyaml/loader.d	89.61% <100%> (ø)	⬆️
source/dyaml/queue.d	98.87% <90.9%> (-1.13%)	⬇️

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