Merge Node constructors

[Herringway]

Previously, the constructors for Node were separate because they couldn't all share the same attributes - nothrow, @safe, etc. Since attributes are automatically inferred for templates, this is no longer necessary.

Other notes:

• There were no examples for scalar Node construction, this has been fixed.
• Nodes may now be constructed using the contents (and nothing else) of other nodes.

[codecov[bot]]

Codecov Report

Merging #146 into master will increase coverage by <.01%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #146      +/-   ##
==========================================
+ Coverage   93.37%   93.38%   +<.01%     
==========================================
  Files          31       31              
  Lines        4331     4337       +6     
==========================================
+ Hits         4044     4050       +6     
  Misses        287      287

Impacted Files	Coverage Δ
source/dyaml/node.d	91.66% <100%> (+0.06%)	:arrow_up:

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