Add Separate Nodes for Predicate and RelationDeclaration to auth_logic/ast.h

[aferr]

At the moment the authorization logic ast refers to the definitions of predicate and relationDeclaration from the datalog ir (in datalog/program.h). This was done for efficiency and to avoid code reuse. However, by adding equivalent classes to ast.h we can cleanly separate the datalog IR from the authoriation logic ast and the visitor that runs on the ast can completely traverse the AST without creating a dependency on the datalog IR.