convert Resolver to struct

[Herringway]

There's still some little ugly parts left lying around (such as the .resolver() methods found in Loader and Dumper) that will be removed/refactored later.

As a bonus, users no longer need to construct their own Resolver unless they wish to exclude all default tags. There may also be an improvement when loading/dumping multiple files, as the default regexes now only need to be compiled once per thread.

One step closer to closing #215.

[codecov]

Codecov Report

Merging #229 into master will decrease coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #229      +/-   ##
==========================================
- Coverage   94.89%   94.88%   -0.02%     
==========================================
  Files          28       28              
  Lines        4269     4260       -9     
==========================================
- Hits         4051     4042       -9     
  Misses        218      218

Impacted Files	Coverage Δ
source/dyaml/test/emitter.d	96.49% <ø> (-0.18%)	⬇️
source/dyaml/test/constructor.d	95.5% <ø> (-0.05%)	⬇️
source/dyaml/composer.d	95.83% <ø> (-0.04%)	⬇️
source/dyaml/dumper.d	94.28% <100%> (-0.06%)	⬇️
source/dyaml/loader.d	89.18% <100%> (-0.43%)	⬇️
source/dyaml/resolver.d	94.33% <100%> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update b63ea1a...1cf7016. Read the comment docs.