SRL abstract syntax

[afs]

Part of #1194

I would start with a concrete definition of the abstract syntax.

Something like

Term ::= RDFTerm | Var
Triple ::= (Term, IRI | Var, Term)

RuleElement ::= TriplePatternElem(Triple)
            | ConditionElem(Expr)
            | NegationElem(Triple+)
            | AssignmentElem(Var, Expr)

RuleBody ::= RuleElement*
RuleHead ::= Triple+

Rule ::= (RuleHead, RuleBody)

RuleSetMember ::= Rule | DataBlock | RuleImport
RuleSet ::= RuleSetMember*