Implement range interface for Loader, remove old opApply interface

[Herringway]

opApply sucks. let's kill it

[Herringway]

Not sure whether or not the version should be bumped with this. it's technically a changed interface, but old code should still work

[codecov[bot]]

Codecov Report

Merging #196 into master will decrease coverage by 0.03%. The diff coverage is 92.59%.

@@            Coverage Diff             @@
##           master     #196      +/-   ##
==========================================
- Coverage   93.85%   93.81%   -0.04%     
==========================================
  Files          29       29              
  Lines        4313     4303      -10     
==========================================
- Hits         4048     4037      -11     
- Misses        265      266       +1

Impacted Files	Coverage Δ
source/dyaml/composer.d	96.46% <ø> (-0.21%)	:arrow_down:
source/dyaml/test/representer.d	96.29% <100%> (ø)	:arrow_up:
source/dyaml/test/compare.d	91.66% <100%> (ø)	:arrow_up:
source/dyaml/test/errors.d	81.81% <100%> (ø)	:arrow_up:
source/dyaml/loader.d	89.15% <90.47%> (-1.55%)	:arrow_down:

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[ghost]

The question with tags and DUB is rather if the patch or the minor part of the tag has to be changed.

• patch : no breakage allowed
• minor: breakage allowed (because of the "~>" operator often used in receipts)

But as long as no tag is pushed there's no problem at all. Put this in the 0.8 milestone seems wise and safe.