(PUP-3114) Specify the Resource Expression

This changes the Resource Expression spec:

* The type expression may be a QualifiedName, a QualifiedReference or
  an AccessExpression with a left expression being a QualifiedReference.
* The type expression must evaluate to a CatalogEntry without title.
* The AttributeFromHash operation is now specified to be allowed 
  multiple times.
* The grammar for ResourceDefaults have been updated with a specific
  description rather than listing the exceptions from the regular
  Resource Expression.
(PUP-3117) Specify that *=> can only by used once per resource body

While it is required that attribute names are unique, it is extra
confusing to use multiple *=> entries. Instead, the hash should
be transformed / merged / entries removed and used once. Without this
requirement it is hard to figure out what multiple entries mean.
Merge pull request #18 from hlindberg/PUP-3117_resource-expression

(PUP-3117) Update spec about resource expressions