Creates initial abstract syntax tree data structure (#11)

* Creates intial abstract syntax tree datastructure

* Updates pull request template

go/ast/ast.go

@@ -0,0 +1,26 @@
+package ast
+
+type Node interface {
+	tokenLiteral() string
+}
+
+type Statement interface {
+	Node
+	statementNode()
+}
+
+type Expression interface {
+	Node
+	expressionNode()
+}
+
+type Program struct {
+	Statements []Statement
+}
+
+func (p *Program) tokenLiteral() string {
+	if len(p.Statements) > 0 {
+		return p.Statements[0].tokenLiteral()
+	}
+	return ""
+}