implement YAML schemas

[Herringway]

This is laying some important groundwork for YAML 1.2+ support. Thoughts on the API are welcome.

[codecov]

Codecov Report

Merging #251 into master will decrease coverage by 0.04%.
The diff coverage is 96.55%.

@@            Coverage Diff             @@
##           master     #251      +/-   ##
==========================================
- Coverage   94.97%   94.93%   -0.05%     
==========================================
  Files          28       28              
  Lines        4139     4126      -13     
==========================================
- Hits         3931     3917      -14     
- Misses        208      209       +1

Impacted Files	Coverage Δ
source/dyaml/dumper.d	96.8% <100%> (+0.03%)	⬆️
source/dyaml/loader.d	89.7% <100%> (-0.15%)	⬇️
source/dyaml/composer.d	95.72% <100%> (ø)	⬆️
source/dyaml/resolver.d	89.58% <89.47%> (-3.86%)	⬇️

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[tom-tan]

It would be nice if users can access each SchemaRule to build their own schemas.
For example, we may want to build JSON Schema or other custom schemas by picking up some SchemaRules from YAML11Schema.