lang: Add class and include statements

This adds support for the class definition statement and the include
statement which produces the output from the corresponding class.

The classes in this language support optional input parameters.

In contrast with other tools, the class is *not* a singleton, although
it can be used as one. Using include with equivalent input parameters
will cause the class to act as a singleton, although it can also be used
to produce distinct output.

The output produced by including a class is actually a list of
statements (a prog) which is ultimately a list of resources and edges.
This is different from functions which produces values.

lang/interfaces/ast.go

@@ -68,6 +68,7 @@ type Expr interface {
 type Scope struct {
 	Variables map[string]Expr
 	//Functions map[string]??? // TODO: do we want a separate namespace for user defined functions?
+	Classes map[string]Stmt
 }
 
 // Empty returns the zero, empty value for the scope, with all the internal
@@ -76,6 +77,7 @@ func (obj *Scope) Empty() *Scope {
 	return &Scope{
 		Variables: make(map[string]Expr),
 		//Functions: ???,
+		Classes: make(map[string]Stmt),
 	}
 }
 
@@ -85,13 +87,18 @@ func (obj *Scope) Empty() *Scope {
 // we need those to be consistently pointing to the same things after copying.
 func (obj *Scope) Copy() *Scope {
 	variables := make(map[string]Expr)
+	classes := make(map[string]Stmt)
 	if obj != nil { // allow copying nil scopes
 		for k, v := range obj.Variables { // copy
 			variables[k] = v // we don't copy the expr's!
 		}
+		for k, v := range obj.Classes { // copy
+			classes[k] = v // we don't copy the StmtClass!
+		}
 	}
 	return &Scope{
 		Variables: variables,
+		Classes:   classes,
 	}
 }