Introduced the evaluator (and integrated it with the REPL)

It supports integer and boolean literals, prefix expressions and infix
expressions with integer operands.

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

pkg/object/object.go

@@ -0,0 +1,47 @@
+package object
+
+import "fmt"
+
+type ObjectType string
+
+const (
+	INTEGER_OBJ = "INTEGER"
+	BOOLEAN_OBJ = "BOOLEAN"
+	NULL_OBJ    = "NULL"
+)
+
+type Object interface {
+	Type() ObjectType
+	Inspect() string
+}
+
+type Integer struct {
+	Value int64
+}
+
+func (i *Integer) Type() ObjectType {
+	return INTEGER_OBJ
+}
+func (i *Integer) Inspect() string {
+	return fmt.Sprintf("%d", i.Value)
+}
+
+type Boolean struct {
+	Value bool
+}
+
+func (b *Boolean) Type() ObjectType {
+	return BOOLEAN_OBJ
+}
+func (b *Boolean) Inspect() string {
+	return fmt.Sprintf("%t", b.Value)
+}
+
+type Null struct{}
+
+func (n *Null) Type() ObjectType {
+	return NULL_OBJ
+}
+func (n *Null) Inspect() string {
+	return "null"
+}