Parse call expressions and gets rid of the TODOs in parse let and return.

Signed-off-by: jmug <u.g.a.mariano@gmail.com>

pkg/ast/call.go

@@ -0,0 +1,31 @@
+package ast
+
+import (
+	"bytes"
+	"strings"
+
+	"code.jmug.me/jmug/interpreter-in-go/pkg/token"
+)
+
+type CallExpression struct {
+	Token     token.Token // The ( token
+	Function  Expression
+	Arguments []Expression
+}
+
+func (ce *CallExpression) expressionNode() {}
+func (ce *CallExpression) TokenLiteral() string {
+	return ce.Token.Literal
+}
+func (ce *CallExpression) String() string {
+	var out bytes.Buffer
+	out.WriteString(ce.Function.String())
+	out.WriteString("(")
+	args := []string{}
+	for _, arg := range ce.Arguments {
+		args = append(args, arg.String())
+	}
+	out.WriteString(strings.Join(args, ", "))
+	out.WriteString(")")
+	return out.String()
+}