Add planner, though it's so simple.

One of the biggest concerns at the moment is about circlic imports.  We
(will) have tightly coupled sub-systems, and it makes convenient to separate
name spaces.  However, Go doesn't allow circlic imports, which may be hit
as the system gets complicated.

src/bigpot/planner/planner.go

@@ -0,0 +1,67 @@
+package planner
+
+import (
+	"bigpot/parser"
+//	"bigpot/system"
+)
+
+type Node interface {
+	Visit(func(Node))
+}
+
+type PlanRoot struct {
+	CommandType		parser.CommandType
+	Plan			Node
+	RangeTables	 []*parser.RangeTblEntry
+}
+
+type Planner interface {
+	Plan(query parser.Query) *PlanRoot
+}
+
+type PlannerImpl struct {
+
+}
+
+type Plan struct {
+	LeftTree Node
+	RightTree Node
+}
+
+type SeqScan struct {
+	Plan
+	TargetList []*parser.TargetEntry
+	RangeTable *parser.RangeTblEntry
+	rel			uint32
+}
+
+func (planner *PlannerImpl) Plan(query parser.Query) *PlanRoot {
+	root := PlanRoot{}
+
+	root.CommandType = query.CommandType
+
+	root.Plan = Node(makeSeqScan(query.TargetList, query.RangeTables[0]))
+	/* TODO: deep copy */
+	root.RangeTables = query.RangeTables
+
+	return &root
+}
+
+func makeSeqScan(tlist []*parser.TargetEntry, rte *parser.RangeTblEntry) *SeqScan {
+	scan := &SeqScan{
+		TargetList: tlist,
+		RangeTable: rte,
+	}
+
+	return scan
+}
+
+func (node *Plan) Visit(callBack func(node Node)) {
+	callBack(node)
+	if node.LeftTree != nil {
+		node.LeftTree.Visit(callBack)
+	}
+	if node.RightTree != nil {
+		node.RightTree.Visit(callBack)
+	}
+}