(PUP-2898) Add 4x style resource expressions

This changes the following:

• Assignment has lower precedence than resource expressions
• Local defaults are supported in resource instantiation expressions
• A Resource body may have a new expression AttributesOperation that

must evaluate to a hash.

• Stricter validation of resource titles
• Changes to TreeModelDumper wrt how BlockExpression is rendered (to

improve debugging output). As a consequence many tests needed
adjustments.

• Tests that previously passed have been modified because of stricter

validation.

This also includes maintenance. Removing dead code/fixing stale comments
and documentation.