dev: better pp and inspect

[flavorjones]

What problem is this PR intended to solve?

Improve output of inspect and pp for some classes: AttributeDecl, ElementContent, ElementDecl, and EntityDecl

These classes were either:

• inheriting PP::Node and overriding #inspect but not #pretty_print, or
• not inheriting PP::Node where they could be to simplify things

Additionally, PP::Node#inspect and PP::Node#pretty_print no longer munge the names of inspected attributes (e.g., attribute_nodes appears as-is instead of as attributes), and if there is only a single inspect_attributes, we omit the key name which makes the XML declarations' output nicer.

Have you included adequate test coverage?

Err, no, but these changes should only impact debugging.

Does this change affect the behavior of either the C or the Java implementations?

N/A