Implemented const keyword

tests/programs/const.q

@@ -0,0 +1,12 @@
+const num {
+	one   1
+	two   2
+	three 3
+}
+
+main() {
+	assert num.one == 1
+	assert num.two == 2
+	assert num.three == 3
+	assert num.one + num.two == num.three
+}

tests/programs_test.go

@@ -31,6 +31,7 @@ var programs = []struct {
 	{"binary", "", "", 0},
 	{"octal", "", "", 0},
 	{"hexadecimal", "", "", 0},
+	{"const", "", "", 0},
 	{"array", "", "", 0},
 	{"escape-rune", "", "", 0},
 	{"escape-string", "", "", 0},