Adds let statements to AST (#12)

go/ast/ast.go

@@ -1,5 +1,7 @@
 package ast
 
+import "github.com/abasnfarah/interpreter/go/token"
+
 type Node interface {
 	tokenLiteral() string
 }
@@ -18,6 +20,29 @@ type Program struct {
 	Statements []Statement
 }
 
+type Identifier struct {
+	Token token.Token
+	Value string
+}
+
+func (i *Identifier) expressionNode() {}
+
+func (i *Identifier) tokenLiteral() string {
+	return i.Token.Literal
+}
+
+type LetStatement struct {
+	Token token.Token
+	Name  *Identifier
+	Value Expression
+}
+
+func (l *LetStatement) statementNode() {}
+
+func (l *LetStatement) tokenLiteral() string {
+	return l.Token.Literal
+}
+
 func (p *Program) tokenLiteral() string {
 	if len(p.Statements) > 0 {
 		return p.Statements[0].tokenLiteral()